UNIVERSITY  OF  CALIFORNIA 
AT  LOS  ANGELES 


GIFT  OF 

MRS, JOHN   G.SHEDD 


fUL 


SHOP  TESTS 


ELECTRIC  CAR  EQUIPMENT 


FOR   INSPECTORS   AND   FOREMEN 


BY 
EUGENE  C.  PARHAM,  M.E. 

AND 

JOHN  C.  SHEDD,  PH.D. 


NEW  YORK 
McGRAW   PUBLISHING   COMPANY 

239  WEST  39TH  STREET 
1909 


Copyright.  1909 

by  the 

McGRAW  PUBLISHING  COMPANY 
New  York 


PREFACE. 

This  book  is  the  first  of  two  books  designed  to 
cover  in  a  practical  manner  the  testing  of  electric 
car  equipment  with  such  instruments  and  other 
facilities  as  may  be  available  or  obtainable  in  a 
car  house.  An  effort  is  made  to  so  present  the 
subject  that  the  instructions  and  information  given 
can  be  profitably  used  even  if  not  entirely  under- 
stood. To  this  end  simple  explanations,  illustra- 
tions and  practical  examples  are  freely  used.  The 
appended  questions,  it  is  believed,  will  efficiently 
rehearse  the  readers'  knowledge  of  the  informa- 
tion contained  in  the  text.  In  the  methods  given 
refinement  is  at  times  sacrificed  to  practicability 
with  the  object  of  showing  how  results  are  obtained 
rather  than  how  they  might  be.  As  the  subject 
covers  new  ground,  at  least  in  the  method  of 
presentation,  the  writers  feel  that  suggestions 
from  readers  would  be  especially  valuable  and  in- 
vite the  readers'  cooperation. 

THE  AUTHORS. 


206525 


CONTENTS. 

PAGE 

Current  Measurements 1 

Ammeter  Method 1 

Connections 1 

Reading  Large  Currents 2 

Voltmeter  Resistance  Method 6 

Wattmeter  Method 8 

Wattmeter 8 

Watt-hour  Meter 9 

Voltage  Measurements 11 

Voltmeter  Method 11 

Connections 11 

Reading  High  Voltages 12 

Ammeter  Resistance  Method 17 

Reducing  Effective  Voltage 18 

Lamp  Method 20 

Resistance  Measurements 21 

Introduction 21 

Resistances  in  Series 21 

Resistances  in  Parallel 22 

Conductor  Resistances 30 

Voltmeter- Ammeter  Method 30 

Voltmeter  Resistance  Method 33 

Ammeter  Resistance  Method 37 

Differential  Method 38 

Wheatstone  Bridge  Method 40 

Home-made  Slide  Wire  Bridge 41 

Ohmmeter 44 

v 


vi  CONTENTS. 

Insulation  Resistance 48 

Introduction 48 

Voltmeter  Method 49 

Lamp-circuit  Method 58 

Bell-Circuit  Method 61 

Magneto  Method 62 

High  Voltage  Insulation  Test 64 

Miscellaneous  Tests 67 

Armature  Tests 67 

Bar-to-Bar  Tests 67 

Lead-to-Bar  Tests 73 

Telephone  Tests 74 

Differential  Voltmeter  Test 77 

Transformer  Test 79 

Lathe  Test 82 

Spinning  Test 82 

Field-Connection  Test 85 

Locating  Charged  and  Grounded  Car  Parts. .  87 

General  Testing  Precautions 92 

Help  to  the  Injured 94 

Reviving  Shocked  Persons 94 

Relieving  Burns 96 

Questions 97 

Index..  ..116 


RULES. 

NO.  PAGE 

1.  Constant  for  shunted  ammeter 4 

2.  To  measure  current  by  drop  through  given  resist- 

ance    6 

3.  To  measure  current  with  a  wattmeter 8 

4.  To  measure  current  with  a  watt-hour  meter 9 

5.  Maximum  e.m.f.  to  be  measured  with  two  voltme- 

ters in  series 13 

G.  Constant  of  a  voltmeter  with  a  multiplier 15 

7.  To  determine  multiplier  resistance  by  calculation.  15 

8.  To  determine  multiplier  resistance  by  experiment .  16 

9.  To  measure  e.m.f.  with  ammeter  and  known  resist- 

ance      17 

10.  The  total  resistance  of  resistances  in  series 22 

1 1 .  The  equivalent  resistance  of  equal  resistance  in 

parallel 23 

12.  The  equivalent  resistance  of  any  two  resistances 

in  parallel 24 

13.  The  equivalent  resistance  of  any  number  of  re- 

sistances in  parallel 25 

14.  To  determine  the  amount  of  resistance  to  be  con- 

nected in  parallel  with  a  given  resistance  to  ob- 
tain a  given  total  resistance 26 

15.  To  determine  the  amount  of  resistance  to  be  con- 

nected in  parallel  with  any  number  of  known 
parallel  resistances  to  give  a  certain  total  re- 
sistance    27 

16.  To  measure  resistance  with  a  voltmeter  and  am- 

meter      30 

17.  To    measure    resistance    with    a    voltmeter    and 

known  resistance 34 

18.  To    measure    resistance    with    an    ammeter    and 

known  resistance 37 

19.  To  measure  resistance  with  a  Wheatstone  bridge . .  43 

20.  To    measure    insulation    resistance    with    a    volt- 

meter      50 

21.  To  measure  insulation  resistance  with  a  voltmeter .  51 

22.  To  measure  insulation  resistance  with  a  voltmeter .  58 

23.  Maximum  test  current  for  given  size  of  armature  . .  71 

24.  Safe  test  current  for  motor  of  unknown  rating ....   72 

25.  To  test  for  ground  and  for  live  parts  with  a  volt- 

meter    88 


Shop  Tests  on  Electric  Car 
Equipment 

CURRENT  MEASUREMENTS. 

AMMETER  METHOD. 

CONNECTIONS. 

1.  To  measure  current  with  an  ammeter,  con- 
nect the  meter  in  series*  with  the  circuit  in  which 
the  current  is  to  be  measured: — thus  to  measure  the 
current  of  a  series  motor  or  dynamo,  connect  in  at 
X,  Y  or  Z,  Fig.  1 ;  as  this  is  a  simple  series  circuit 
the  current  in  all  parts  is  the  same.  In  a  shunt 


FIELD 


-  1     ARMATURE 
X 

FIG.  1. 

dynamo  the  current  divides  between  the  external 
and  field  circuits;  in  a  shunt  motor,  between  the 
armature  and  field  circuits;  in  either  case  to  mea- 
sure: (a)  total  current,  connect  the  meter  at  X, 
Fig.  2;  (6)  armature  current,  at  Y;  field  current, 
at  Z.  Never  break  the  circuit  at  the  meter  and 
be  certain  that  the  meter  +  post  connects  to  the 
*  See  articles  20  and  21. 


2  SHOP  TESTS. 

+  side  of  the  circuit,  to  avoid  slamming  the  needle 
to  the  wrong  side  of  0.  In  no  case  must  the  meter 
be  subjected  to  current  exceeding  its  rating; 
such  currents  cannot  be  read  and  are  liable  to 
injure  the  instrument.  Ammeter  resistance  is  so 
low  that  its  insertion  in  a  circuit  has  a  negligible 
effect. 

READING  LARGE  CURRENTS. 
2.  Two    Ammeters    in    Parallel.     Currents    ex- 
ceeding the  rating  of  a  single  meter  can  be  indi- 
cated on   two   meters   in   parallel,   provided  their 


ARMATURE 

FIG.  2. 

relative  resistances  are  such  that  the  current  divides 
between  them  proportionally ;  otherwise  one  needle 
will  be  thrown  off  the  scale  and  the  other  indicate 
less  than  it  should.  For  two  meters,  A  and  B, 
to  indicate  current  equal  to  the  sum  of  their  ratings, 
the  resistance  of  A  must  be  as  many  times  that  of 
B  as  the  current  rating  of  B  is  times  that  of  .4. 
For  a  50-ampere  meter  and  a  150-ampere  meter  to 
indicate  200  amperes,  the  resistance  of  the  50- 
ampere  meter  must  be  3  times  that  of  the  150- 
ampere  meter.  Proportional  sharing  of  current 
between  such  small  resistances  can  be  effected 


CURRENT  MEASUREMENTS.  3 

better  by  experiment  than  calculation,  because 
the  resistance  of  a  poor  connection  can  easily 
exceed  that  of  the  meter.  Current  division  be- 
tween parallel  meters  is  actually  effected  by  vary- 
ing the  binding  screw  pressures  on  them  and  it 
will  be  necessary  to  do  this  even  when  the  meters 
have  equal  resistances  and  ratings.  A  better  way 
is  to  increase  the  length  of  the  leads  of  the  meter 
that  takes  more  than  its  part;  thus  in  Fig.  3,  if 
meter  A  takes  more  than  its  share,  loosen  its  bind- 


FIG.  3. 

ing  screws  and  tighten  those  of  B;  or  lengthen 
leads  a. and  b  or  shorten  c  and  d.  These  devices 
are  permissible  in  testing,  but  not  as  permanent 
connections,  for  they  waste  energy  and  cause  heat- 
ing that  might  eventually  start  a  fire. 

3.  Shunting  an  Ammeter.  The  rating  of  an  am- 
meter can  be  increased  by  a  shunt  adjusted  to  take 
a  known  part  of  the  current ;  thus  if  a  meter  has  in 
parallel  with  it,  a  resistance  equal  to  its  own,  each 
will  take  half  the  current :  when  using  this  shunted 
meter  to  indicate  current,  every  reading  must  be 


SHOP  TESTS. 


multiplied  by  2  to  get  the  total  current;  here  2  is 
called  the  constant  and  it  applies  to  only  this  par- 
ticular shunt  adjustment. 

4.  With  a  second  meter  it  is  easy  to  adjust  a 
shunt  to  any  shunting  power.  Thus  in  Fig.  4,  the 
difference  in  the  readings  of  meters  A  and  B,  is 
the  current  in  shunt  S;  if  A  indicates  100  and  B,  25 
amperes,  the  current  in  5  is  100  —  25  =  75  am- 
peres. As  the  meter  shunted  indicates  but  a 
quarter  of  the  total  current,  the  constant  or  multi- 
plier is  4.  In  all  cases  the  constant  to  be  used 

SHUNTED  METER 

a1 


FIG. 


in  a  shunt-ammeter-adjustment  can  be  obtained 
by  rule  1. 

Rule  1.  To  get  the  constant  of  a  shunted  am- 
meter, divide  the  total  current  by  the  current  in  the 
shunted  meter. 

Example  1.  With  a  main  current  of  150  amperes, 
the  shunted  ammeter  reading  is  50  amperes.  What 
is  the  constant  and  how  is  it  to  be  used? 

Solution  1.  By  rule  1,  150-J-50  =  3,  constant; 
when  measuring  currents,  every  reading  must  be 
multiplied  by  3  to  get  the  total  current. 

5.  Where  no  main  current  ammeter  is  available, 


CURRENT  MEASUREMENTS.  5 

adjustment  on  a  circuit  of  variable  voltage  is  te- 
dious, but  can  be  made  with  the  connections  of 
Fig.  5.  Here  R  is  a  variable  resistance;  A,  the 
meter,  to  be  shunted  and  S  is  the  shunt,  including 
switch  K  for  cutting  it  in  and  out.  With  K  open, 
A  indicates  total  current;  with  K  closed,  S  takes 
part  of  the  current.  The  length  of  lead  X  can  be 
varied  by  drawing  it  through  post  p.  To  make  an 
adjustment,  open  K  and  adjust  the  current  to  a 
value  less  than  the  rating  of  A;  then  repeatedly 
close  K,  loosen  post  p,  change  the  length  of  X  or 


Y,  tighten  p  and  open  K,  until  the  deflection  with 
K  open  is  2,  3  or  4  times  that  with  it  closed.  As  a 
rule  the  constant  is  made  2,  the  length  of  X  being 
changed  until  closing  K  halves  the  reading  of 
ammeter  A. 

6.  Where  the  meter  is  wanted  for  but  few  read- 
ings it  is  unnecessary,  to  get  a  whole-number 
constant.  For  example,  if,  on  closing  K  the  first 
time,  the  deflection  falls  from  150  to  62  and  trials 
show  this  relation  to  be  correct,  then  the  constant, 
2.419  (150-^62  =  2.419)  could  be  used.  For  a 


6  SHOP  TESTS. 

great  number  of  readings  the  labor  of  multiplying 
by  this  fractional  constant  would  be  so  great  as 
to  warrant  a  whole  number. 

"Notes.  Conductors  X  and  Y  should  be  of  the 
largest  size  to  be  easily  drawn  through  post  p, 
otherwise  the  current  is  liable  to  heat  them  and 
thereby  change  the  resistance  and  alter  the  con- 
stant. 

A  shunt  adjusted  for  one  meter  will  have  a  dif- 
ferent constant  when  used  with  another.  Con- 
stants must  in  each  case  be  found  by  trial. 

VOLTMETER  RESISTANCE  METHOD. 

7.  To  measure  current  with  a  voltmeter  and 
known  resistance,  connect  the  resistance  in  series 
with  the  conductor  in  which  current  is  to  be  meas- 
ured and  in  parallel  with  the  voltmeter.  Take  a 
reading  and  apply  rule  2. 

Rule  2.  To  calculate  current  from  the  drop  across 
a  known  resistance,  divide  the  drop  across  the  known 
resistance  by  the  resistance. 

Example  2.  The  current  taken  at  full  speed  on 
level  track  and  a  voltage  of  550,  by  a  21-ton  car 
equipped  with  four  50-hp.  motors  must  be  meas- 
ured with  a  voltmeter  and  a  known  resistance. 
How  can  it  be  done? 

Solution  2.  In  series  with  the  overhead  switch 
or  circuit-breaker,  connect  a  50-hp.  railway  motor 
field  coil  to  be  used  as  a  standard  resistance,  R, 
Fig.  6,  and  in  parallel  with  the  coil  connect  a  switch 
K,  to  be  closed  except  when  taking  a  reading; 


CURRENT  MEASUREMENTS.  7 

connect  the  voltmeter  across  the  standard  coil. 
Let  the  car  reach  full  speed,  open  K  and  take  a 
reading.  Assume  the  standard  field  coil  to  measure 
0.035  ohm  and  the  drop  across  it  to  be  5.25  volts, 
apply  rule  2: 

5.25-7-0.035  =  150  amperes.     Ans. 

Notes.  Without  K,  R  will  get  hot,  its  resistance 
and  drop  will  increase  and  indicate  more  current 
than  exists.  Without  K,  R  must  be  measured 
after  each  reading  and  this  value  used  in  the  cal- 


culation. If  only  the  approximate  current  is 
wished,  take  the  drop  in  one  of  the  car  motor  field 
coils,  the  approximate  resistance  of  which  is  known, 
and  apply  rule  2  to  get  the  current  of  one  motor; 
this  multiplied  by  the  number  of  motors  is  ap- 
proximately the  total  current  taken  by  the  car. 

To  get  close  results,  R  should  be  a  standard  re- 
sistance not  much  affected  by  temperature  changes. 
The  voltmeter  should  be  low  reading  so  that  a 
small  drop  will  give  a  large  deflection. 


8  SHOP  TESTS. 

Example  3.  The  field  resistance  of  a  shunt-wound 
shop  motor  is  125  ohms  cold  and  140  ohms  hot; 
on  a  500  volt  line  what  is  the  field  current:  (a) 
when  the  motor  is  first  started?  (6)  after  the 
field  windings  are  hot? 

Solution  3.  (a)  Field  current  at  start  =  500  + 125 
=  4  amperes.  Ans.  (a). 

(b)    Field    current    after    a    run  =  500-^140  =  3.57 
amperes.     Ans.  (6). 

WATTMETER  METHODS. 

WATTMETER. 

8.  A  wattmeter  shows  the  instantaneous  value 
of  the  rate  at  which  energy  is  absorbed  in  the 
circuit  with  which  the  meter  is  connected.  As  it 
indicates  the  product  of  volts  and  amperes,  i.e., 
watts,  if  its  indication  be  divided  by  the  volts, 
the  result  will  be  the  amperes. 

Rule  3.  To  measure  direct  current  with  an  indi- 
cating wattmeter,  connect  its  series  coil  in  series 
with  the  conductor  in  which  current  is  to  be  meas- 
ured and  apply  a  known  voltage  to  the  shunt  coil. 
Divide  any  watt  reading  by  the  known  voltage  and 
the  result  is  the  current. 

Example  4.  The  current  of  a  25-hp.  motor  driv- 
ing mill  shafting  is  to  be  measured;  the  source  of 
standard  voltage  is  28  storage  cells  (60  volts  total 
voltage)  ordinarily  used  to  operate  multiple-unit 
train-control  circuits.  Apply  a  wattmeter  to  this 
test. 

Solution   4.  Connect   the   wattmeter  series   coil 


CURRENT  MEASUREMENTS.  9 

in  series  with  the  motor  circuit,  as  in  Fig.  7,  and 
the  shunt  coil  across  the  battery.  Close  K  and  K' 
and  read  the  meter;  calling  the  reading  1,200,  by 
rule  3. 

1 ,200  -4-  60  =  20  amperes.     Ans. 

Note.  As  the  meter  indication  depends  on  the 
product  of  the  volts  and  amperes  to  which  the 
meter  is  subjected,  the  reading  will  be  lower  than 
if  the  meter  were  so  connected  as  to  indicate  the 


WATT  METER 

FIG.  7. 

power  of  the  motor,  because  the  battery  voltage 
is  less  than  the  voltage  across  the  motor  terminals. 

WATT-HOUR  METER. 

9.  A  watt-hour  meter  (often  erroneously  called 
recording  or  integrating  wattmeter)  shows  the 
watt-hours  of  energy  absorbed  in  the  time  the 
record  is  taken.  Dividing  this  watt-hour  record 
by  the  time  in  hours  or  fractions  of  an  hour  gives 
the  average  absorption  rate  or  power  in  watts. 

Rule  4,  To  measure  current  with  a  watt  hour 


10  SHOP  TESTS. 

meter,  divide  the  meter  record  by  the  time  in  hours, 
then  by  the  known  voltage  acting. 

Example  5.  Apply  a  watt-hour  meter  to  the 
measurement  in  example  4. 

Solution  5.  Connect  the  meter  as  in  Fig.  7,  the 
known  voltage  being  connected  to  the  meter  ar- 
mature; take  a  reading,  close  K  and  K'  and  let 
the  meter  run  for  10  minutes  (J  hour),  then  open 
K  and  K'  and  take  the  reading.  Subtract  the  first 
reading  from  the  second  to  get  the  watt-hours, 
in  this  particular  case  200,  and  apply  rule  4: 

200-5-1  =  1,200 
and  1,200 -=-60  =  20  amperes.     Ans. 

Note.  Ordinarily  a  watt-hour  meter  gives  no 
information  as  to  the  voltage  and  current  acting 
on  it  but  gives  their  product  multiplied  by  time. 
Sometimes  it  is  interesting  to  know  the  average 
current  of  the  test ;  this  can  be  gotten  from  an  am- 
meter connected  in  series  with  the  watt-hour  meter 
or  by  taking  periodical  readings  of  a  voltmeter  in 
parallel  with  it,  then  dividing  the  watt-hour  meter 
record  in  watts  by  the  average  voltage.  Where 
a  circuit  has  fairly  constant  voltage,  the  average 
current  can  be  approximated  by  dividing  the  watt- 
hour  record  by  the  voltage. 


VOLTAGE  MEASUREMENTS. 

VOLTMETER  METHOD. 

CONNECTIONS. 

10.  To  measure  voltage  or  potential  drop  (p.d.) 
with  a  voltmeter,  connect  lines  from  the  meter 
to  the  points  between  which  the  p.d.  is  to  be  read, 
the  +  post  of  the  meter  being  connected  to  the 
+  side  of  the  circuit.  In  Fig.  8,  if  the  volt-lines  be 
touched  to  T  and  G,  the  meter  will  indicate  line 
voltage;  if  touched  to  T  and  a,  the  p.d.  in  resist- 
ance R  1  will  be  indicated;  contact  with  a  and  b 

T         ftl     a.     R2       b      R3 


X  TROLLEY  RESISTANCES       1 

>  +     -  GROUND  -TL 

VOLT  Q  METER  G 

FIG.  8. 

will  show  the  p.d.  in  R  2  and  so  on.  With  5  lamps 
in  series  across  550  volts,  as  in  Fig.  9,  the  p.d.  in 
any  lamp  is  indicated  by  applying  the  volt-lines 
across  the  terminals  of  that  lamp.  Such  a  test 
will  show  the  drop  to  vary  from  lamp  to  lamp 
because  the  resistance  per  lamp  varies.  Standing 
on  the  ground  and  touching  the  +  side  of  lamp  5 
will  give  a  shock  due  to  the  p.d.  in  one  lamp — 
about  110  volts;  on  touching  the  ground  and  the  + 
11 


12  SHOP  TESTS. 

side  of  lamp  4,  the  shock  will  be  due  to  the  drop 
in  2  lamps — 220  volts,  and  so  on,  contact  with 
the  ground  and  +  side  of  lamp  1  giving  a  shock 
due  to  the  p.d.  in  5  lamps — line  voltage. 

11.  The  resistance  of  a  voltmeter  is  high,  and 
its  current  small.  When  used  to  indicate  the  volt- 
age across  an  open  circuit,  as  in  Fig.  10,  the  meter 
bridges  the  gap  and  there  is  a  current;  but  the 
current  is  negligibly  small.  The  voltmeter  so 
placed  is  in  series  with  the  rest  of  the  circuit, 
but  the  meter  resistance  is  comparatively  so  great 


GROUND^." 


that  practically  the  total  p.d.  takes  place  across 
it.  In  connecting  a  voltmeter  to  indicate  the  drop 
in  a  resistance,  as  in  Fig.  6,  the  meter  is  in  parallel 
with  the  resistance  in  which  the  drop  is  to  be 
indicated ;  the  effect  of  placing  a  resistance  in  par- 
allel with  another  is  to  reduce  the  resistance  be- 
tween the  two  points  touched;  but  the  meter  re- 
sistance is  so  high  as  to  have  but  little  effect. 

READING  HIGH  VOLTAGES. 

12.  Two  Voltmeters  in  Series.    Voltages  exceed- 
ing the  rating  of  one  meter  may  be  indicated  on 


VOLTAGE  MEASUREMENTS.  13 

two  connected  in  series  as  in  Fig.  11.  Two  volt- 
meters so  connected  cannot  indicate  a  voltage 
equal  to  the  sum  of  their  ratings  unless  their  re- 
sistances are  proportional  to  their  ratings.  Thus, 
for  two  500- volt  meters  to  read  1,000  volts,  their 

BATTERY 


VOLTAMETER 

FIG.  10. 

resistances  must  be  equal  so  that  each  will  get  a 
p.d.  of  500  volts,  otherwise  the  needle  of  the  lower- 
resistance  meter  will  indicate  less  than  maximum 
while  that  of  the  other  will  go  off  the  scale.  Sim- 
ilarly, 600- volt  and  150- volt  meters  in  series  will 

VOLTMETERS  IN  SERIES 


FIG.  11. 

not  read  to  750  volts  unless  the  resistance  of  the 
former  is  4  times  that  of  the  latter.  The  maximum 
voltage  readable  on  two  meters  of  known  resist- 
ance and  rating,  can  be  obtained  by  applying  rule  5. 
Rule  5.  To  determine  the  maximum  voltage 


14  SHOP  TESTS. 

readable  on  two  voltmeters  of  known  resistance 
and  rating  in  series,  add  the  meter  resistances, 
multiply  by  the  higher  rating  and  divide  by  the 
higher  resistance. 

Example  6.  What  is  the  maximum  voltage  read- 
able on  a  600-volt,  80,000-ohm  voltmeter  con- 
nected in  series  with  a  500-volt,  60,000-ohm  volt- 
meter? 

Solution  6.  From  rule  5, 

60,000  +  80,000  =  140,000 
140,000  X  600  =  84,000,000 
and  84,000,000 -=-80,000  =1,050  volts.     Ans. 


VOLT  \7  METER 


13.  Voltmeter  and  Multiplier.  The  rating  of  a 
voltmeter  can  be  increased  by  connecting  a 
resistance  in  series  as  in  Fig.  12.  A  resistance  so 
used  is  called  a  multiplier.  Multipliers  are  fur- 
nished on  request  but  cannot,  without  extra  cal- 
culation, be  used  with  a  meter  other  than  the  one 
for  which  adjustment  was  made.  If  in  series  with 
a  meter  is  connected  a  resistance  equal  to  its  own, 
the  applied  voltage  will  divide  equally  between 
them;  as  the  meter  indicates  but  half  the  total 
voltage,  the  reading  must  be  multiplied  by  2; 
here  2  is  the  constant  of  the  particular  multiplier 


VOLTAGE  MEASUREMENTS.  15 

and  voltmeter  used.  In  any  case,  the  constant  of 
a  voltmeter  and  multiplier  can  be  gotten  by  ap- 
plying rule  6. 

Rule  6.  To  get  the  constant  of  a  voltmeter  and 
multiplier  of  known  resistance,  add  their  resistances 
and  divide  by  resistance  of  the  voltmeter. 

Example  7.  The  range  of  a  500-volt,  60,000-ohm 
voltmeter  is  to  be  increased  with  a  49,000-ohm 
multiplier  belonging  to  another  voltmeter.  What 
is  the  constant  of  the  new  arrangement? 

Solution  7.  From  rule  6, 

60,000  +  49,000  =  109,000 
and  109,000-5-60,000=1.817.     Ans. 

Notes.  Were  the  meter  to  indicate  500  volts 
the  total  potential  difference  would  be  500X1.817 
=  909  volts.  An  indication  of  453  volts  would 
mean  a  total  e.m.f.  of  453X1.816  =  823  volts,  and 
so  on. 

The  maximum  rating  of  a  voltmeter  is  indicated 
by  the  highest  value  marked  on  its  scale.  The 
resistance  can  be  found  on  the  certificate  pasted 
on  the  inner  side  of  the  sliding  box  cover,  or  can 
be  ascertained  from  the  instrument  maker  by  for- 
warding the  meter  number. 

14.  Determination  of  Multiplier.  The  series  re- 
sistance required  to  increase  a  voltmeter  range  by 
a  certain  amount,  can  be  gotten  by  rule  7. 

Rule  7.  To  determine  the  multiplier  resistance 
required  to  increase  a  voltmeter  range  by  a  certain 
amount,  divide  the  new  range  by  the  old,  to  get  the 


16  SHOP  TESTS. 

constant;  then  multiply  the  meter  resistance  by  the 
constant  and  subtract  the  meter  resistance. 

Example  8.  The  maximum  rating  of  a  500-volt, 
60,000-ohm  voltmeter  is  to  be  increased  to  550 
volts.  What  multiplier  resistance  must  be  used? 

Solution  8.  The  new  maximum  being  550  and 
the  old,  500  volts,  the  constant  is  550-5-500  =  1.1: 

60,000X1.1=66,000 
and          66,000  -  60,000  =  6,000  ohms.     Ans. 

15.  Experimental  Determination  of  the  Voltmeter 
Constant.  The  constant  of  a  voltmeter  and  multi- 
plier of  known  or  unknown  resistance  can  be  ex- 


MULTIPLIER  VOLT   METER 


V   K  GROUND  ' 

FIG.  13. 

perimen tally  determined  as  follows:  To  a  voltage 
less  than  the  maximum  rating  of  the  meter,  con- 
nect the  meter,  multiplier  and  switch  K,  as  in 
Fig.  13.  With  K  closed,  the  meter  indicates  line 
voltage;  with  K  open,  the  multiplier  is  in  series. 
The  test  consists  in  taking  readings  with  K  open, 
and  closed,  then  applying  rule  8. 

Rule  8.  To  determine  experimentally  the  con- 
stant of  a  voltmeter  and  multiplier  from  a  deflec- 
tion with  the  multiplier  cut  out  and  another  with 
the  multiplier  cut  in,  divide  the  first  deflection  by 
the  second. 


VOLTAGE  MEASUREMENTS.  17 

Example  9.  The  deflection  with  the  multiplier  cut 
out,  Fig.  13,  is  450;  with  it  cut  in,  405;  find  the 
constant  for  that  arrangement. 

Solution  9.  From  rule  8, 

450  +  405  =  1 . 1 1 1 ,  Constant.     Ans. 

Note.  Twenty-eight  16-c-p.  incandescent  lamps 
in  series  would  be  about  right  (28X220  =  6,160 
ohms)  for  the  multiplier  of  example  8.  As  it  is 
not  exact,  however,  the  constant  would  be  found 
by  rule  8. 

Example  10.  The  constant  of  a  voltmeter  with  28 

AMMETER 
KNOWN  RESISTANCE 


VARIABLE 

RESISTANCE  G  A 

FlG.  14. 

lamps  in  series,  is   1.11.     To  what  total  voltage 
would  a  deflection  of  500  correspond? 
Solution  10.  From  the  note,  Art.  13, 

500X1.11-555  volts.     Ans. 

AMMETER  RESISTANCE  METHOD. 

16.  To  get  the  p.d.  in  a  conductor  of  known 
resistance,  connect  an  ammeter  in  series,  Fig.  14, 
establish  a  current,  take  a  reading  and  apply  rule  9. 

Rule  9.  To  calculate  the  p.d.  in  a  conductor  of 
known  resistance  carrying  direct  current,  multiply 
the  current  by  the  resistance. 

Example  11,  The  resistance  of  two  G.  E.  800  field. 


18  SHOP  TESTS. 

coils  in  series  is  0.624  ohm.     What  voltage  do  they 
consume  when  the  motor  carries  60  amperes? 

Solution  11.  From  rule  9, 

0.624X60  =  37.44  volts.     Ans. 

Example  12.  A  G.  E.  1,200  motor  shunt-wound 
to  run  shop  shafting  has  a  field-circuit  resistance 
of  330  ohms ;  one  field  coil  is  to  be  dried  by  current. 
On  a  line  voltage  of  500  and  in  series  with  a  resist- 
ance of  200  ohms,  to  what  p.d.  will  the  field  coil 
be  subjected? 

Solution  12.  The  G.  E.  1,200  motor  has  two 
field  coils  connected  in  series,  so  the  resistance  per 
coil  is  165  ohms.  The  field  coil  and  outside  resist- 
ance in  series,  then  measure  165  +  200  =  365  ohms. 
From  rule  2,  500  volts  will  establish  through  365 
ohms,  a  current  of  (500-7-365)  1.3  amperes.  From 
rule  9,  then,  the  p.d.  in  the  coil  is 

1.3X165  =  213.5  volts.     Ans. 

Note.  In  operation  500  volts  are  applied  to  2 
coils  in  series,  so  each  gets  a  p.d.  of  250  volts; 
213.5  volts  is,  then,  perfectly  safe. 

REDUCING  EFFECTIVE  VOLTAGE. 
17.  The  voltage  on  a  device  can  be  changed  by 
regulating  that  of  the  supply,  but  this  is  imprac- 
ticable except  on  a  dynamo  used  exclusively  for 
testing.  The  common  method  is  to  put  resistance 
in  series  to  consume  part  of  the  applied  voltage, 
the  remainder  acting  on  the  device  in  question. 
For  testing,  the  voltage  can  be  varied  by  using 
storage  cells,  the  number  in  series  being  adjusted 


VOLTAGE  MEASUREMENTS.  19 

to  suit  the  work ;  but  storage  cells  are  seldom  avail- 
able. A  good  way  to  get  any  voltage  from  0  to 
full  line-voltage  is  by  the  use  of  proportion  lines. 
In  Fig.  15,  R  is  a  resistance  connected  from  trolley 
to  ground  through  switch  K;  if  R  is  composed  of 
similar  units  such  as  cast  iron  resistance  grids,  it 
is  easy  to  determine  how  many  units  must  be  in- 
cluded, to  get  a  desired  voltage.  Supposing  R  to 
be  a  series  of  200  G.  E.  grids,  section  26510,  0.1 
ohm  each,  the  200  grids  will  measure  20  ohms 
and  the  current  on  closing  K  will  be  500-^-20  =  25 


VOLTAMETER 

FIG.  15. 

amperes.  As  there  is  a  drop  of  500  volts  in  200 
grids,  the  drop  per  grid  is  500-^200  =  2.5  volts. 
If  the  volt-lines,  here  called  proportion  lines,  be- 
cause they  tap  a  certain  proportion  of  the  total 
voltage,  be  placed  across  1  grid,  the  meter  will 
indicate  a  drop  of  2.5  volts  and  if  the  lines  be 
placed  across  10  grids,  the  drop  indicated  will  be 
25  volts.  In  any  case  if  one  proportion  line  is  ap- 
plied to  trolley  or  ground  and  the  other  to  an  inter- 
mediate grid,  the  included  voltage  is  the  number 
of  grids  between  them,  times  the  drop  per  grid. 
If  R  is  not  composed  of  similar  units,  the  desired 


20  SHOP  TESTS. 

proportion  of  the  total  voltage  can  be  first  found 
with  the  voltmeter,  and  the  device  to  be  energized 
then  connected  to  the  points  so  found.  Propor- 
tion lines  are  especially  useful  in  experimental 
work  and  in  adjusting  voltmeter  multipliers  by 
trial.  Thus  in  Fig.  13  the  line  voltage  might  ex- 
ceed the  range  of  the  meter,  in  which  case  closing 
K  would  injure  the  instrument;  the  proportion 
lines  afford  a  safe  voltage  for  making  an  adjust- 
ment that  would  hold  for  the  higher  voltages. 

LAMP  METHOD. 

18.  Voltage  can  be  measured  approximately  by 
finding  out  how  many  similar  incandescent  lamps 
of  known  voltage  and  in  series,  it  will  light  to  full 
brilliancy;  this  found,  the  total  voltage  acting  is 
that  marked  on  each  lamp  base  multiplied  by  the 
number  of  lamps  in  series;  if  the  test  voltage 
lights  twenty  110- volt  lamps,  its  value  is  20X110 
=  2,200  volts.  Were  the  lamps  50-volt  lamps,  the 
voltage  acting  would  be  20  X  50  =  1 ,000.  The  lamp 
method  of  indicating  voltage  is  applicable  to  di- 
rect-current (d.c.)  and  alternating-current  (a.c.) 
circuits.  In  a.c.  insulation  testing,  lamps  are  used 
on  the  high  tension  side  of  the  transformer  to 
indicate  the  test  voltage.  Series  of  5-volt  or  10- volt 
lamps  are  useful  where  only  d.c.  instruments  are 
available  and  a.c.  voltages  must  be  indicated;  a 
voltage  of  500  will  light  50  ten-volt  lamps  to  nor- 
mal brilliancy  and  the  voltage  can  be  indicated 
within  a  narrow  margin  of  error. 


RESISTANCE  MEASUREMENTS. 

INTRODUCTION. 

19.  Resistance  is  the  opposition  that  substances 
offer  to  the  electric  current  through  them.  Insu- 
lators offer  great  resistance;  conductors,  compara- 
tively little.  A  small  conductor  has  more  resistance 
than  a  large  one  of  the  same  length  and  material; 
a  long  conductor,  more  than  a  short  one  of  the 
same  size  and  material.  Considering  two  or  more 
conductors,  their  combined  resistance  depends  on 
how  they  are  connected:  if  connected  to  increase 


FIG.  16. 

the  cross  section  of  the  current  path,  their  combined 
resistance  will  be  less  than  that  of  one  conductor; 
if  connected  to  increase  the  length  of  the  path, 
their  combined  resistance  will  be  greater  than  the 
resistance  of  one  conductor. 

RESISTANCE  IN  SERIES. 

20.  Two  or  more  conductors  so  connected  that 
the  current  must  pass  through  all  in  succession, 
are  in  series;  in  Fig.  16,  conductors  A  and  B  are 
in  series;  the  only  path  of  the  current  is  through 
each  to  reach  the  other  and  a  break  in  either  will 


22  SHOP  TESTS. 

stop  the  current.  If  A  and  B  are  each  1  ohm,  the 
resistance  from  A  to  b  is  2  ohms;  hence  rule  10. 

Rule  10.  To  get  the  resistance  of  two  or  more 
conductors  connected  in  series,  add  their  individual 
resistances. 

Example  13.  On  the  first  notch  of  a  series  par- 
allel controller,  the  car  wiring,  controller  contacts, 
starting  coil  and  car  motors  are  in  series.  If  the 
starting  coil  measures  5.52  ohms,  the  wiring  and 
contacts,  0.3  ohm  and  the  two  motors  in  series,  0.8 
ohm,  what  is  the  resistance  of  the  current  path 
from  trolley  wheel  to  rail? 


Solution  13.  From  rule  10, 

5.52  +  0.3  +  0.8  =  6.62  ohms.     Ans. 

Note.  This  shows  that  in  calculating  the  re- 
sistance of  the  starting-coil  for  a  car,  other  circuit 
resistances  must  be  considered. 

RESISTANCES  IN  PARALLEL. 

21.  Conductors  so  connected  that  a  break  in 
one  does  not  stop  the  current  in  the  others,  are  in 
parallel  or  multiple.  In  Fig.  17  conductors  .4  and  B 
are  in  parallel;  current  can  traverse  either  after 


RESISTANCE  MEASUREMENTS.  23 

the  other  has  been  removed.  As  the  resistance 
of  a  large  conductor  is  less  than  that  of  a  small 
one  of  the  same  length  and  material  and  as  two 
conductors  in  parallel  are  equivalent  to  a  single  one 
of  the  same  length  and  material  and  of  their  com- 
bined cross  section,  the  resistance  of  two  conductors 
in  parallel  is  less  than  that  of  either  alone.  If  A 
and  B  are  each  1  ohm,  the  resistance  from  a  to  b 
is  half  an  ohm. 

22.  Parallel  Resistance  of  Equal  Resistances.  The 
parallel  resistance  of  any  number  of  equal  resist- 
ances can  be  calculated  from  rule  11. 

Rule  11.  To  get  the  resistance  of  two  or  more 
equal  resistances  in  parallel,  divide  the  resistance 
of  one  of  them  by  their  number. 

Example  14.  A  16-c-p.,  110-volt  incandescent 
lamp  measures  about  220  ohms.  Calculate  the 
resistance  of  5  such  lamps  connected  in  parallel. 

Solution  14.  By  rule  11, 

220  -T-  5  =  44  ohms.     Ans. 

Example  15.  Five  110-volt,  32-c-p.  incandescent 
lamps  in  series,  measure  about  550  ohms.  What  is 
the  resistance  of  7  such  series  in  parallel? 

Solution  15.  By  rule  11,    (Fig.  18) 

550  H-  7  =  78.57  ohms.     Ans. 

Note.  Conductors  so  connected  that  some  are 
in  series  and  others  are  in  parallel,  are  said  to  be 
in  parallel-series  or  multiple  series.  In  Fig.  18  the 
individual  lamps  in  a  row  are  in  series — unscrewing 
a  lamp  will  extinguish  a  row.  The  rows,  however, 


24 


SHOP  TESTS. 


are  in  parallel  with  each  other;  extinguishing  one 
row  will  not  stop  the  current  in  the  others. 

23.  Parallel  Resistance  of  Any  Two  Resistances. 
The  parallel  resistance  of  two  equal  or  unequal  re- 
sistances can  be  calculated  by  rule  12. 

Rule  12.  To  get  the  parallel  resistance  of  any 
two  known  resistances,  divide  their  product  by 
their  sum. 

Example  16.  A  110- volt,  16-c-p.  incandescent 
lamp  measures  220  ohms;  a  110- volt,  32-c-p. 
lamp,  110  ohms.  What  is  their  parallel  resistance? 

LAMPS 

\      /  \      /  If  N      t  \     / 


FIG.  18. 

Solution  16.  By  rule  12, 

220X110  =  24,200;  and  220  +  110  =  330  and  24,200 
-i-  330  =  73.33  ohms.     Ans. 

Example  17.  A  G.  E.  1,000  field  coil  measures 
0.44  ohm;  a  shunt  for  it,  1.1  ohms;  what  is  their 
parallel  resistance? 

Solution  17.  By  rule  12, 

0.44X1.1=0.484  and  0.44  +  1.1  =  1.54  and  0.484 -^ 
1.54  =  0.314  ohm.     Ans. 


RESISTANCE  MEASUREMENTS.  25 

24.  Parallel  Resistance  of  Two  or  More  Unequal 
Resistances.  The  customary  method  of  calculating 
the  parallel  resistance  of  more  than  two  unequal 
resistances,  is  complicated  and  hard  to  remember. 
Rule  13  is  easy  to  retain. 

Rule  13.  To  get  the  parallel  resistance  of  more 
than  two  unequal  resistances,  find  the  parallel  re- 
sistance of  any  two  of  them  by  rule  12;  then  the  par- 
allel resistance  of  this  result  and  a  third  resistance; 
then  the  parallel  resistance  of  this  and  a  fourth  re- 
sistance and  so  on  until  all  of  the  resistances  have 
been  so  used. 

Example  18.  Five  conductors  measuring  1,  2, 
3,  4  and  5  ohms  are  connected  in  parallel.  What 
is  their  parallel  resistance? 

Solution  18.  The  parallel  resistance  of  the  first 
two  conductors  =  (2  X  1)  -f-  (2  + 1)  =  2  -f-  3  =  f  ohm. 

The  parallel  resistance  of  this  result  and  the 
third  conductor  =  (§  X  3)  -5-  (§  +  3)  =  6/3  ^  3§  =  6/3  ^ 
11/3  =  6/3X3/11  =  18/33  =  6/11  ohm. 

The  parallel  resistance  of  this  result  and  the 
fourth  conductor  =  (6/ 1 1  X  4)  -=-  (6/1 1  +  4)  =  24/1 1 
-T- 50/11  =24/11X11/50  =  264/550  =  12/25  ohm. 

The  parallel  resistance  of  this  result  and  the 
fifth  conductor  =  (12/25  X  5)  +  (12/25  +  5)  =  60/25 
-j- 137/25  =  60/25  X 25/1 37  =  60/137  =  0.437  ohm. 
Ans. 

Example  19.  Five  conductors  have  individual  re- 
sistances 6,  7,  8,  9  and  10  ohms.  What  is  their 
parallel  resistance? 

Solution  19.  The  parallel  resistance  of  first  two 


26  SHOP  TESTS. 

conductors  =  (6  X  7)  -^  (G  +  7)  =  42  -=- 13  =  42/13  = 
3.23  ohms. 

The  parallel  resistance  of  this  result  and  the 
third  conductor  =  (3.23  X  8)  -=-  (3.23  48)=  25.84  -^ 
11.23  =  2.30  ohms. 

The  parallel  resistance  of  this  result  and  the 
conductor    4  =  (2.3  X  9)  •*-  (2.3  +  9)  =  20.7  -5- 1 1 .3  = 
1.83  ohms. 

The  parallel  resistance  of  this  result  and  con- 
ductor 5  =(1.83X10)  4-  (1.83  +  10)  =  18.3  -=-11. 83  = 
1.54  ohms.  Ans. 

25.  Calculation  of  Resistance  to  be  Added  to 
Give  a  Certain  Parallel  Resistance.  It  may  be  de- 
sired to  know  what  resistance  must  be  connected 
in  parallel  with  an  existing  resistance  to  give  a 
certain  parallel  resistance.  (See  rule  14). 

Rule  14.  To  get  the  resistance  to  be  connected 
in  parallel  with  a  known  resistance  to  give  a  desired 
parallel  resistance,  multiply  the  existing  known  and 
desired  parallel  resistances  together  and  then  divide 
by  their  difference. 

Example  20.  An  existing  conductor  measures  5 
ohms;  it  is  desired  to  connect  in  parallel  with  it  a 
conductor  of  resistance  such  that  the  parallel  re- 
sistance shall  be  1.75  ohms.  What  must  be  the 
resistance  of  the  added  conductor? 

Solution  20.  Here  the  existing  resistance  is  5 
ohms  and  the  desired  parallel  resistance  is  1.75 
ohms.  By  rule  14,  then, 

5  X  1.75  =  8.75  and  5  -  1.75  ==  3.25  and  8.75  -=-  3.25  = 
2.692  ohms.     Ans. 


RESISTANCE  MEASUREMENTS.  27 

26.  Where  an  existing  conductor  is  composed 
of  several  parallel  conductors  of  known  individual 
but  unknown  parallel  resistance  and  another  con- 
ductor is  to  be  added  to  reduce  the  parallel  resist- 
ance to  a  certain  value,  find  the  existing  parallel 
resistance  by  rule  13;  then  considering  this  result 
as  the  existing  resistance,  apply  rule  14.  Hence 
rule  15. 

Rule  15.  To  get  the  resistance  of  a  conductor 
that  is  to  be  connected  in  parallel  with  existing 
parallel  conductors  to  bring  their  final  parallel 
resistance  to  a  certain  value,  find  the  existing  par- 
allel resistance  by  rule  13  and  use  it  as  the  existing 
resistance  in  applying  rule  14. 

Example  21.  Two  conductors  measuring  3  and  7 
ohms  are  in  parallel  and  it  is  desired  to  connect 
in  parallel  with  them,  a  third  conductor  of  resist- 
ance such  that  the  final  parallel  resistance  of  the 
three  conductors  shall  be  1  ohm.  What  must  the 
added  conductor  measure? 

Solution  21.  As  one  conductor  measures  3  and 
the  other  7  ohms,  the  existing  parallel  resistance 
is,  by  rule  13,  (3X7)  -T-  (3  +  7)  =21  -i- 10  =  2.1  ohms. 
Calling  2.1  ohms  the  existing  resistance  and  ap- 
plying rule  14,  2.1X1  =  2.1  and  2.1-1  =  1.1  and 
2.1 -r  1.1  =  1.9  ohms.  Ans. 

Note.  1.9  ohms  is  the  resistance  of  the  con- 
ductor to  be  connected  in  parallel  with  the  3  and 
7-ohm  conductors  to  give  a  final  parallel  resistance 
of  1  ohm.  To  test  the  result,  suppose  that  it  is 
desired  to  find  the  parallel  resistance  of  the  1.9, 


28  SHOP  TESTS. 

3  and  7-ohm  resistances;  by  rule  13  the  parallel 
resistance  of  the  first  two  conductors  is  ( 1.9X3) -r- 
(1.9  +  3)  =5.7  -T-  4.9  =1.16  ohms  and  the  parallel  re- 
sistance of  this  result  and  the  third  conductor  is, 
(1. 16 X 7)  -f-  (1.16  +  7)  =8.42 -8.16  =  1.0  ohm.  Ans. 

Example  22.  The  resistance  of  a  starting  coil 
for  four  40-h.p.  motors  on  a  20-ton  car  to  operate 
on  550  volts,  is  to  be  4  ohms  maximum.  The  4  ohms 
are  represented  by  a  single  section  in  circuit  on  the 
first  notch.  On  notch  2,  a  second  section  cut  in 
parallel  with  the  first,  reduces  the  circuit  resist- 
ance to  2  ohms;  in  the  same  manner,  on  the  third 
notch  the  resistance  is  reduced  to  1  ohm;  on  the 
fourth,  to  0.5  ohm;  on  the  fifth,  to  0.3  ohm;  on  the 
sixth,  to  0.1  ohm,  and  on  the  seventh  notch  a  short- 
circuiting  wire  reduces  the  starting  coil  resistance 
to  0.  It  is  thus  seen  that  six  resistance  sections  are 
needed  and  it  is  desired  to  know  what  must  be  the 
resistance  of  each  that  it  may  produce  the  stated 
change  when  it  is  introduced?  The  short-circuiting 
wire  is  not  to  be  considered  as  a  section. 

Solution  22.  First  notch.  In  circuit  here  is  one 
section  of  4  ohms — a,  Fig.  19. 

Second  notch.  Here  is  cut  in  parallel,  section  b, 
such  that  the  resistance  of  a  and  b  in  parallel  shall 
be  2  ohms.  The  resistance  existing  is  4  ohms 
and  the  desired  parallel  resistance,  2  ohms,  so, 
by  rule  14,  the  resistance  of  added  coil  b  is,  (4  X2)  •*• 
(4—2)  =  8-=-2  =  4  ohms  =  resistance  of  coil  or 
section  b. 

Third  notch.  The  resistance  existing  on  notch  2 


RESISTANCE  MEASUREMENTS.  29 

is  2  ohms  and  the  third  notch  must  add  a  section  c, 
such  that  the  parallel  resistance  of  a,  b  and  c, 
shall  be  1  ohm.  The  resistance  of  section  c  is, 
then,  (2Xl)n-(2  — 1)  =  2-=-l  =  2  ohms  =  resist- 
ance of  coil  c. 

Fourth  notch.  The  existing  resistance  on  notch  3 
is  1  ohm  and  notch  4  must  add  a  coil  d  such  that 
the  parallel  resistance  of  a,  b,  c  and  d  shall  be  0.5 
ohm.  The  resistance  of  section  d  must  be,  then. 


FIG.  19. 

(1X0.5) -T- (1  —  0.5)  =  0.5-^0.5  =  1  ohm  =  resist- 
ance of  section  d. 

Fifth  notch.  The  existing  resistance  on  notch  4 
is  0.5  ohm  and  the  fifth  notch  is  to  add  a  coil  e 
such  that  the  parallel  resistance  of  a,  b,  c,  d  and  e 
shall  be  0.3  ohm.  The  resistance  of  added  coil,  e, 
is,  (0.5X0.3) -«- (0.5 -0.3)  =  0.15-^-0.2  =  0.75  ohm 
=  resistance  coil  e. 

Sixth  notch.  The  resistance  existing  on  notch  5 
is  0.3  ohm  and  the  sixth  notch  is  to  add  a  section  / 


30  SHOP  TESTS. 

such  that  the  parallel  resistance  of  a,  b,  c,  d,  e 
and  /  shall  be  0.1  ohm.  The  resistance  of  /  is, 
then,  (0.3X0.1)-*- (0.3 -0.1)  =  0.03^0.2  +  0.15 
ohm,  resistance  of  /. 

Seventh  notch.  As  the  starting  coil  is  entirely 
short-circuited  here,  no  resistance  section  is  added, 
but  instead,  a  heavy  wire  19. 

Note.  The  advantages  of  this  parallel  type  of 
resistance  coil  over  the  better  known  series  type, 
are  that  the  current  carrying  capacity  increases  as 
the  current  does  and  an  open  circuit  in  a  resistance 
connection  affects  operation  only  on  the  notch  in 
which  the  open  circuit  is  directly  involved. 

CONDUCTOR  RESISTANCE. 
VOLTMETER  AMMETER  METHOD. 

27.  The  usual  shop  method  of  measuring  con-' 
ductor  resistance  is  with  an  ammeter  in  series 
with  the  conductor  and  a  voltmeter  across  it.  The 
circuit  includes  a  variable  resistance  to  regulate 
the  current,  unless  the  test  resistance  is  known  to 
be  sufficient.  The  test  is  to  pass  a  safe  current 
through  the  circuit,  take  simultaneous  voltmeter 
and  ammeter  readings,  then  apply  rule  16. 

Rule  16.  To  get  the  resistance  of  a  conductor 
from  simultaneous  readings  of  a  voltmeter  in 
parallel  and  an  ammeter  in  series  with  the  con- 
ductor, divide  the  voltmeter  reading  by  the  ammeter 
reading. 

Example  23.  In  Fig.  20,  x  is  the  starting  coil 
for  a  20-ton  car  operated  by  four  40-hp.  motors 


RESISTANCE  MEASUREMENTS. 


31 


and  a  series-parallel  controller;  describe  how  its 
resistance  may  be  measured  with  a  voltmeter  and 
ammeter. 

Solution  23.  In  the  test  from  which  the  example 
was  taken  the  voltmeter  indicated  120  volts  and 


AMMETER 


FIG.  20. 

the  ammeter,  30  amperes;  by  rule  16,  then,  the 
resistance  was, 

120 -i- 30  =  4  ohms.     Ans. 

Example  24.  The  resistance  of  the  top  and  bot- 
tom sections  of  a  set  of  electric  heaters  is  to  be 


AMMETER       HEAT  SWITCH 


TOP  SECTIONS 


'.METER 


FlG.  21. 


How 


measured  with  a  voltmeter  and  ammeter, 
can  this  be  done? 

Solution  24.  Fig.  21  shows  car  heater  connec- 
tions; on  notch  1  of  the  switch,  the  top  sections 
in  series  are  active;  on  notch  2,  the  bottom  sec- 


32  SHOP  TESTS. 

tions  are  in  series.  The  ammeter  is  in  series  with 
the  heater  trolley  tap  and  the  voltmeter  is  across 
trolley  and  ground.  In  this  test  the  current 
reading  on  notch  1  was  6  amperes  and  the  voltage, 
486.  On  notch  2  the  current  was  10  and  the 
voltage,  490. 

(a)   Resistance  top  sections  =  486-7-6  =  81  ohms. 

Ans.  (a). 

(6)    Resistance    bottom    sections  =  490-^10  =  49 

ohms.     Ans.  (b). 

Note.  On  notch  3  the  top  sections  in  series  and 


V.  METER 


FIG.  22. 

the  bottom  sections  in  series  are  in  parallel  with 
each  other.  The  parallel  resistance  of  the  two 
sections  is,  by  rule  12,  (81  X  49) -^  (81  +  49)  =  3969 
-^- 130  =  30.5  ohms.  Also,  by  rule  2,  the  current 
on  a  500  volt  circuit  would  be,  500 -J- 30.5  =  16.4 
amperes,  showing  that  the  current  taken  to  heat 
an  electric  car  is  very  considerable. 

Example  25.  Wanted,  to  measure  with  ammeter 
and  volt-meter,  the  resistance  per  lamp  and  total 
resistance  of  a  lamp  circuit  of  five  16-c-p.,  110- volt 
incandescent  lamps  connected  in  series. 


RESISTANCE  MEASUREMENTS.  33 

Solution  25.  Connect  the  lamp  circuit  and  a 
low  reading  ammeter  in  series  as  in  Fig.  22.  With 
the  volt-meter  take  the  drop  in  the  whole  lamp 
circuit,  as  indicated  by  the  full  line,  then  the  drop 
in  each  lamp,  as  indicated  with  the  dotted  lines 
to  lamp  4.  In  each  case  take  the  corresponding 
ammeter  reading  and  tabulate  the  results  as 
shown. 

Note.  The  measured  resistance  of  the  circuit  is 
less  than  the  sum  of  the  measured  resistances  of 
the  individual  lamps,  because  voltage  variations 
prevented  strictly  simultaneous  readings;  and  be- 
cause the  ammeter  (25  amperes)  permitted  ex- 
cessive error  in  reading  currents  so  small  as  0.5 
ampere.  Decided  difference  in  the  brilliancies  of 
lamps  4  and  5  foretold  the  decided  difference  in 
their  resistances. 

Volts    Amperes    Ohms 
All,       505  +  0.50  =  1,010.0 

1,  101  +  0.49  =      206.1 

2,  99  +  0.48=      206.2 

3,  103  +  0.50=     206.0 

4,  102  +  0.49=      208.1 

5,  100  +  0.52  =      192.3 

VOLTMETER — TRESISTANCE  METHOD. 
28.  When  voltage  is  applied  to  resistances  con- 
nected in  series  the  greatest  p.d.  is  found  in  the 
greatest  resistance,  the  least  in  the  least  resistance 
and  so  on,  the  voltage  distributing  itself  according 
to  the  distribution  of  resistance.  On  this  fact  is 
based  rule  17. 


34  SHOP  TESTS. 

Rule  17.  To  measure  an  unknown  resistance 
with  a  volt-meter  and  known  resistance,  establish 
a  current  through  the  known  and  unknown  re- 
sistances in  series  and  read  the  drop  on  each  at  the 
same  current  value;  multiply  the  knovm  resistance 
by  the  drop  in  the  unknown  and  divide  by  the  drop 
in  the  known. 

Example  26.  The  resistance  of  a  field  coil  is  to 
be  measured  with  a  volt-meter  and  a  similar  field 
coil  that  is  sound.  How  can  it  be  done? 

Solution   26.  Connect   a   switch   K,   a   variable 


FlG.  23. 

resistance  R,  a  standard  coil  5,  and  test  coil  X  in 
series,  as  in  Fig.  23,  and  use  a  safe  value  of  current. 
Take  the  drop  in  the  standard  coil,  then  in  the  test 
coil,  then  in  the  standard  coil  again,  to  see  that 
the  current  has  not  varied,  then  apply  rule  17. 
In  one  case  the  standard  measured  0.08  ohm  and  its 
drop  was  3.5  volts ;  the  test  coil  gave  2  volts  drop : — 
0.08X2  =  0.16  and  0.16-7-3.5  =  0.0457  ohm.  Ans. 
Example  27.  A  set  of  field  coils  is  to  be  mea- 
'sured  in  an  installed  car  motor.  How  can  the 
test  be  made  with  a  volt-meter  and  a  known 
resistance? 


RESISTANCE  MEASUREMENTS,  35 

Solution  27.  Connect  the  motor  field  coils,  a 
switch,  a  variable  resistance  and  a  standard  coil 
in  series ;  use  a  safe  value  of  current,  take  the  drops 
on  the  standard  and  motor  coils.  If  the  motor 
has  four  coils,  their  drop  should  be  four  times  that 
in  the  standard;  if  the  motor  field  drop  is  more  or 
less  than  it  should  be,  all  coils  being  at  the  same 
temperature,  the  faulty  coil  must  be  located  by 
taking  the  drop  on  each.  In  one  test  the  motor 
field  drop  was  10  volts  and  that  in  the  0.08  ohm 
standard,  3  volts.  By  rule  17,  then,  the  re- 
sistance of  the  motor  fields  was,  (0.08X10) -=-3  = 
0.8/3  =  0.266  ohm.  Ans. 

Note.  The  average  resistance  per  coil  (0.266 -f- 
4  =  0.066)  being  too  low,  the  motor  was  opened 
and  the  drop  on  each  coil  taken  and  compared 
with  that  on  the  standard  at  the  same  current. 
The  drops  on  the  standard  and  on  each  of  three  of 
the  motor  coils  was  the  same,  but  on  the  fourth 
coil  it  was  only  1  volt.  As  this  was  entirely  too 
low,  the  insulation  was  ripped  off  and  inspection 
showed  the  coil  to  be  wound  with  too  large  a 
wire. 

Note.  In  testing  field  coils,  it  is  not  customary 
to  work  out  resistances,  as  comparison  of  drops 
suffices. 

Example  28.  With  a  low  reading  volt-meter, 
the  resistance  on  the  last  series  position  of  a  con- 
troller is  to  be  measured.  How  can  it  be  done? 

Solution  28.  The  only  resistance  available  as  a 
standard  was  a  reel  of  527  feet  of  No.  6  B  &  S 


36  SHOP  TESTS. 

copper  wire,  a  wire  table  giving  its  resistance  as 
0.4  ohm  per  1,000  ft.  or  0.0004  ohm  per  ft.  The 
standard  resistance  was,  then,  527X0.0004  =  0.21 
ohm.  A  switch,  the  standard  and  a  variable  re- 
sistance (not  shown)  were  connected  in  series  with 
the  trolley  pole  as  in  Fig.  24.  A  controller  was  put 
on  the  last  series  notch,  the  brake  set  to  prevent 
motion,  current  applied  and  drops  read  from  the 
trolley  wire  to  the  trolley  wheel  and  from  the  trolley 
wheel  to  the  rail.  In  one  test  the  drop  on  the  car 


TROLLEY    WIRE 


COIL 
USED  AS  STAN  OARD 


CONTROLLER 


^-CAR  MOTORS*        G  A. 
FlG.  24. 


circuit  was  60  volts  and  that,  on  the  standard,  6 
volts.  The  car  circuit  resistance  was,  then,  by 
rule  17, 

(0.21X60) -=-6  =  2.10  ohms.     Ans. 

Note.  Overhauling  the  controller,  cleaning  the 
commutators  and  fitting  new  brushes  reduced  the 
resistance  to  1.75  ohms.  The  measurement  in- 
cluded the  resistance  of  the  2  motors  in  series;  to 
get  the  wiring  and  controller  contacts  separately, 
disconnect  the  motor  terminals  and  connect  cor- 
responding cable  terminals. 


RESISTANCE  MEASUREMENTS.  37 

AMMETER-RESISTANCE  METHOD. 

29.  Where  great  accuracy  is  not  required  and 
the  voltage  is  fairly  constant  and  the  resistance  to 
be  measured  is  practically  the  total  resistance  of 
the  circuit,  it  can  be  measured  with  an  ammeter 
and  known  resistance.  Connections  are  as  in  Fig. 
25,  where  K'  is  a  switch  for  short-circuiting  the 
test  resistance.  Readings  are  taken  with  X  in  cir- 
cuit, then  with  X  out  of  circuit.  The  resistance 
value  of  X  is  then  calculated  by  rule  18. 

Rule  18.  To  measure  an  unknown  resistance 
with  an  ammeter  and  a  known  resistance,  connect 


FIG.  25. 

the  meter  and  resistances  in  series,  a  switch  being 
placed  in  parallel  with  the  unknown.  Use  a  safe 
value  of  current  and  take  readings  with  K'  opened 
and  closed.  Divide  the  difference  of  the  readings  by 
the  reading  with  K'  open  and  multiply  by  the  known 
resistance. 

Example  29.  In  Fig.  25,  R  is  a  5-ohm  starting 
coil;  X,  a  starting  coil  for  a  10-ton  car  equipped 
with  two  40-hp.  motors;  K,  a  main  switch  and 
K',  a  switch  for  cutting  X  in  and  out  of  circuit. 
Wanted,  resistance  X. 

Solution  29.  On  closing  K  and  K',  the  current 


206525 


38  SHOP  TESTS. 

is  100  amperes;  opening  K'  reduces  the  current  to 
45.5  amperes.  By  rule  18,  then  100-45.5  =  54.5 
and  54.5-45.5  =  1.19  and  1.19X5  =  6  ohms. 
Ans. 

DIFFERENTIAL  VOLTMETER  METHOD. 
30.  Taking  simultaneously  voltmeter  and  am- 
meter readings  on  a  railway  circuit  is  tedious,  owing 
to  voltage  fluctuations.  In  measuring  resistance 
with  a  voltmeter  and  known  resistance  (Art.  28), 
accuracy  depends  on  getting  both  voltage  readings 


T, 


TEST  LINE 
X-  CAR  RESIST 


with  the  same  current.  A  differential  voltmeter  has 
two  coils  connected  in  opposition;  equal  voltages 
applied  to  them,  hold  the  needle  at  a  central  0. 
The  meter  is  used  as  follows :  In  Fig.  26  the  meter 
has  two  pairs  of  terminals  provided  with  test  lines, 
one  pair  is  connected  to  the  test  resistance  and  the 
other  is  so  connected  to  a  standard  that  one  test 
point  can  be  moved  along  the  resistance  metal. 
Current  is  established  and  the  free  test  point  moved 
along  the  standard  until  the  needle  is  at  0.  Under 
this  condition  the  test  and  standard  resistances  are 


RESISTANCE  MEASUREMENTS.  39 

equal  because  the  current  in  them  and  the  p.d. 
across  them  is  the  same. 

31.  In  an  outfit  for  testing  starting  coils  in  po- 
sition, the  voltmeter  is  a  Weston  differential  in- 
strument reading  to  50  volts  on  either  side  of  0. 
The  standard  resistance  is  a  series  of  220  G.  E. 
grids,  Sec.  26,510,  measuring  22  ohms  or  0.1  ohm 
per  grid.  With  a  water  resistance  the  current  is 
kept  at  a  value  such  that  the  drop  in  the  highest 
resistance  likely  to  be  measured  will  not  exceed 
50  volts.  A  car  resistance,  in  position,  is  measured 
as  follows :  Switch  K,  water  resistance,  standard  R 
and  a  long  test  cable  T\  are  in  series.  A  hook  on 
the  far  end  of  the  test  cable  engages  the  trolley 
wheel  which  is  lowered  from  the  wire.  Test  re- 
sistance volt-lines,  including  switch  K',  normally 
open,  connect  to  one  pair  of  meter  posts  and  the 
volt-lines  from  the  standard  connect  to  the  other 
pair.  The  car  brake  is  set  to  prevent  starting,  the 
car  switch  or  circuit  breaker  closed  and  one  con- 
troller put  on  the  first  notch.  Switch  K  is  closed 
and  the  current  adjusted  until  the  meter  deflection 
does  not  exceed  50  volts  when  the  standard  volt- 
lines  are  at  opposite  ends  of  the  standard  resist- 
ance; on  closing  K'  the  deflection  will  decrease 
owing  to  the  action  of  the  voltmeter  coil  energized 
by  the  p.d.  in  the  car  starting  coil.  Free  test  point 
/  is  then  moved  toward  P  until  the  deflection  be- 
comes 0.  Under  this  condition,  the  resistances  of 
X  and  R  will  be  equal  and  will  be  0.1  ohm  X  num- 
ber of  grids  from  t  to  P.  By  placing  under  the 


40  SHOP  TESTS. 

standard  a  scale  on  which  is  marked  the  resistance 
corresponding  to  each  grid  from  a  to  P,  the  device 
becomes  direct  reading. 

Example  30.  In  measuring  the  resistance  of  a 
starting  coil  on  a  21-ton  car  equipped  with  four 
40-hp.  motors,  the  0  deflection  obtains  with  t  on 
the  negative  end  of  grid  No.  30.  The  coil  measures 
what? 

Solution  30.  By  instructions  of  Art.  30, 

30X0.1  =  3. 00  ohms.     Ans. 

Note.  The  object  of  having  the  total  standard 
resistance  greater  than  that  of  any  starting  coil 
likely  to  be  measured,  is  to  reduce  the  test  current 
to  a  value  that  will  not  cause  excessive  heating. 

WHEATSTONE  BRIDGE  METHOD. 
32.  The  principle  of  all  Wheatstone  bridges  can 
be  understood  in  conjunction  with  Fig.  27,  where 
battery  B  maintains  a  current  through  the  branched 
circuit  a-P-b,  a-Q-b.  The  path  of  the  current  is 
from  a  to  b  through  two  paths,  a-M-X-b,  and 
a-N-R-b.  As  the  p.d.  from  a  to  b  is  fixed,  the  drop 
through  one  path  is  the  same  as  that  through  the 
other  and  for  every  point  in  one,  there  is  a  point 
in  the  other  at  the  same  potential.  If  one  end  of 
a  galvanometer  is  fixed  to  junction  M  X,  as  in  the 
diagram,  and  the  other  end  to  junction  N  R, 
there  will  be  no  deflection  on  closing  K,  if  points 
p  and  q  are  at  the  same  potential.  Since  the  p.d. 
distributes  itself  according  to  the  distribution  of 
resistance,  even  if  p  and  q  are  not  at  the  same  po- 


RESISTANCE  MEASUREMENTS. 


41 


tential  they  can  be  made  so  by  varying  the  resist- 
ance of  one  of  the  arms,  say  R,  or  of  two  arms  at 
once  as  would  be  the  case  were  one  end  of  the 
galvanometer  moved  along  N-R.  In  commercial 
bridges  of  the  plug  type,  R,  called  the  rheostat  arm, 
is  a  resistance  that  can  be  varied  by  inserting  or 
withdrawing  metal  plugs  from  taper  sockets.  In 
slide-wire  bridges,  arms  N  and  R  are  continuous  in 
the  form  of  an  exposed  wire  along  which  one  end 
of  a  galvanometer  or  telephone  circuit  can  be 


FIG.  27. 

moved.  As  plug  bridges  are  less  adapted  to  shop 
use  than  slide-wire  bridges,  only  the  latter  will  be 
considered  here. 

HOMEMADE  SLIDE- WIRE  BRIDGE. 
33.  Fig.  28  shows  a  slide-wire  bridge  easily  made 
at  small  cost  and  with  which  good  work  can  be 
done.  On  a  seasoned,  hard  wood  board  a',  42 
in.  by  6  in.  by  1  in.,  are  mounted  castings  b'  pro- 
vided with  connecting  posts  c'.  A  small  uniform 
german  silver  wire  d  is  stretched  between  the  end 


42 


SHOP  TESTS. 


fittings  and  under  it  is  a  paper  scale  1  meter 
(39.37  in.)  long,  divided  into  1,000  equal  divisions. 
The  battery,  including  a  key  K',  is  connected  to 
junctions  M  N  and  R  X,  as  in  Fig.  27 ;  one  end 
of  the  galvanometer  circuit  is  connected  to  junc- 
tion M  X  and  the  other  to  junction  N  R.  Contact 
Q,  free  to  be  slid  along  wire  N-R  is  the  dividing 
point  between  arms  N  and  R.  Any  movement  of 
Q  changes  the  relative  resistances  of  these  two 
arms.  As  wire  N-R  is  of  uniform  cross  section  the 
resistances  of  arms  N  and  R  need  not  be  known; 


b' 


TELEPHONE 
GALVANOMETER 


FIG.  28. 


but  the  number  of  divisions  in  each  must  be  known 
when  taking  a  reading.  M  is  a  known  resistance 
and  X,  the  resistance  to  be  measured.  Having  the 
connections  made,  a  measurement  consists  in 
closing  key  K  and  sliding  Q  along  the  wire  until 
the  galvanometer  shows  no  deflection.  Where  a 
telephone  is  used,  Q  is  tapped  along  the  wire  until 
a  point  is  reached  where  the  tapping  causes  no 
click  in  the  telephone.  The  nearer  equal  resistances 
M  and  X  are,  the  more  accurate  the  result.  When 
the  balance  point  is  located,  the  bridge  is  said  to  be 


RESISTANCE  MEASUREMENTS.  43 

balanced  and  the  resistance  of  X  in  ohms  can  be 
calculated  by  rule  19. 

Rule  19.  To  get  the  unknown  resistance  corre- 
sponding to  a  balance  on  a  slide-wire  bridge  con- 
nected as  in  Fig.  28,  multiply  standard  resistance 
M  by  ike  number  of  divisions  in  the  diagonally  op- 
posite wire  arm  and  divide  by  the  number  of  divisions 
in  the  adjacent  wire  arm. 

Example  31.  In  measuring  the  resistance  of  an 
arc  headlight  rheostat  a  balance  is  obtained  when 
contact  Q  rests  on  division  615.  To  what  X  re- 
sistance does  this  correspond  when  the  value  of 
M  is  100? 

Solution  31.  Here  the  standard  resistance  is  100 
ohms;  diagonally  opposite  arm  R  has  615  divi- 
sions and  adjacent  arm  N,  385  divisions.    By  rule 
19,  rheostat  resistance  =  100X615/385  =  159.7 
ohms.     Ans. 

Example  32.  A  standard  1-ohm  coil  is  used  at  M 
to  measure  4  G.  E.  1,000  field  coils  connected  in 
series,  a  balance  obtaining  with  contact  Q  on 
division  320.  Wanted  the  resistance  of  the  four 
new  field  coils. 

Solution  32.  Here  the  standard  resistance  is  1 
ohm,  the  opposite  arm  R  has  320  divisions  and  the 
adjacent  arm  N,  680  divisions.  By  rule  19,  then, 
the  resistance  of  the  motor  field  coils  is, 

1X320/680  =  0.47  ohm.     Ans. 

Note.  "Where  a  telephone  is  used  there  will  be 
times  when  the  point  of  silence  cannot  be  exactly 


44  SHOP  TESTS. 

decided,  owing  to  the  click  ceasing  on  a  consider-, 
able  stretch  of  the  wire.  For  example,  it  may 
cease  at  division  590  when  moving  Q  from  the  0 
end  of  the  wire,  and  at  600  when  moving  Q  from 
the  1,000  end;  in  such  cases  find  the  points  at  which 
the  click  ceases  in  both  directions  and  take  the 
point  of  balance  as  half  way  between  them.  In 
the  case  just  supposed  the  balance  point  would  be 
taken  as  division  595. 

OHMMETER. 

34.  An  ohmmeter  is  a  slide-wire  bridge  on  the 
scale  of  which  the  balance-value  of  the  graduations 
is  marked.  In  measuring,  it  is  only  necessary  to 
get  a  balance  and  read  the  number  opposite  the 
balance  point  as  the  value  of  the  unknown  resist- 
ance. The  ohmmeter  is  direct  reading  as  opposed 
to  regular  bridges  in  which  calculation  is  required. 
All  bridges  are  ohmmeters  in  that  they  measure 
ohms  but  the  name  ohmmeter  is  confined  to  those 
bridges  in  which  the  value  of  X  can  be  read  off 
the  scale  on  getting  the  balance.  If,  Fig.  28,  with  a 
certain  standard  resistance  at  M,  different  known 
resistances  be  successively  inserted  at  X,  the  bal- 
ance point  found  and  marked  with  the  balancing 
value  of  X,  the  bridge  becomes  direct  reading  on 
all  points  so  marked.  By  thus  calibrating  20  or  30 
divisions,  then  equally  dividing  intermediate  por- 
tions of  the  scale,  the  whole  scale  becomes  cali- 
brated and  the  slide-wire  bridge  becomes  a  fairly 
accurate  ohmmeter,  for  all  resistances  that  do  not 


RESISTANCE  MEASUREMENTS. 


45 


bring  the  balance  point  near  the  ends  of  the  scale. 
Having  calibrated  the  bridge  for  M  =  l  ohm,  say, 
if  M  be  made  10  ohms,  the  resistance  value  of 
each  scale  division  is  increased  10  times.  To  il- 
lustrate: If  M  and  X  are  both  1  ohm  the  balance 
point  on  the  scale  will  be  at  its  center  or  500  mark, 
which,  accordingly  can  be  marked  1  ohm.  If  M 
and  X  are  made  10  ohms,  the  balance  point  will 
remain  at  500,  but  to  get  the  real  X  value  to  which 
the  balance  is  due,  the  one  ohm  marked  there  must 
be  multiplied  by  10  or  a  second  row  of  numbers 


1000 
too 

10  . 


FIG.  29. 


inscribed.  A  third  row  corresponding  to  an  M 
value  of  100  ohms  and  a  fourth  row  for  M  =  1,000 
ohms  can  be  inscribed  and  the  useful  range  thereby 
enlarged. 

35.  Fig.  20  is  a  top  view  of  an  ohmmeter, 
Fig.  30  showing  the  corresponding  bridge  connec- 
tions. A,  D  and  C  are  connecting  posts  and  A-B-C, 
the  slide-wire  including  brass  end  piece  B  which 
unites  the  two  sections  of  wire.  At  R  are  4  resist- 
ance coils  with  sockets  to  be  engaged  by  plug  P. 
Over  the  divisions  are  four  rows  of  numbers  inked 


46  SHOP  TESTS. 

in  black,  red,  blue  and  brown.  In  testing,  if  P 
plugs  a  hole  marked  black,  red,  blue  or  brown, 
then,  on  getting  a  balance  by  tapping  pointer  S 
along  the  wire,  the  resistance  of  X  can  be  read  di- 
rectly from  the  numbers  of  the  same  color.  T  is  a 
telephone  receiver  provided  with  a  key  K  which 
must  be  closed  before  seeking  the  balance  point 
by  tapping  pointer  5  on  the  wire.  Key  K  is  the 
battery  switch  and  the  intermittent  contact  of 


FIG.  30. 

pointer  5  corresponds  to  switch  Kl  of  Fig.  28. 
On  all  bridges  the  battery  key  must  be  closed  first 
to  allow  the  testing  current  to  become  steady; 
otherwise,  when  testing  field  coils  and  other  re- 
sistances having  what  is  called  self  inductance, 
correct  results  cannot  be  obtained. 

36.  Measurements  with  the  ohmmeter  are  made 
as  follows:  Connect  the  unknown  resistance  to 
A  and  D;  plug  P  into  one  of  the  four  holes;  take 
pointer  5  in  one  hand  and  with  the  other  hold  the 


RESISTANCE  MEASUREMENTS.  47 

'phone  to  the  ear;  close  K  and  tap  5  along  the 
wire  until  the  point  of  silence  or  least  noise  is  found ; 
the  value  of  X  can  then  be  read  on  the  numbers  of 
the  color  indicated  by  the  position  of  plug  P. 
Assuming  X  to  be  such  that  the  balance  point  lies 
on  bar  B,  the  X  value  to  be  read  depends  on  the 
position  of  P;  the  black  hole  has  the  lowest  value 
and  in  the  order  black-red-blue-brown,  each  color 
has  10  times  the  value  of  the  color  preceding.  A 
balance  at  B  with  the  black  hole  would  give  an  X 
value  of  1  ohm;  red  hole,  10  ohms;  blue  hole,  100 
ohms;  brown  hole,  1,000  ohms.  When  measuring, 
select,  by  trial,  the  color  that  brings  the  balance 
point  near  the  centre  of  the  scale,  where  the  divi- 
sions are  farther  apart  and  more  easily  read.  In 
cases  where  the  point  of  silence  cannot  be  exactly 
located,  follow  the  instructions  given  in  the  note 
of  Art.  33.  Where  there  is  an  approximate  idea 
as  to  the  value  of  X,  select  M  to  be  as  nearly  equal 
as  practicable,  as  the  balance  point  will  then  fall 
near  the  scale  center  and  results  will  be  more 
accurate.  The  lower  the  color  used  at  R,  the  lower 
the  unknown  resistance  practicable  to  be  measured 
at  X  and  the  higher  the  color  at  R,  the  higher  the 
resistance  that  can  be  accurately  measured  at  X. 

Example  33.  In  measuring  4  G.  E.  field  coils  in 
the  motor,  the  telephone  ceases  to  click  at  division 
450  and  begins  at  division  470.  To  what  division 
would  the  point  of  silence  correspond? 

Solution  33.  As  the  stretch  of  silence  extends 
from  division  450  to  division  470,  the  silence  point 


48  SHOP  TESTS. 

may   be   taken    half   way   between   division    460, 
giving  the  resistance  as  0.46  ohm. 

INSULATION  RESISTANCE. 
INTRODUCTION. 

37.  Insulation  can  be  tested  with  a  bridge, 
bell  circuit,  magneto,  lamp  circuit,  voltmeter  or 
a.c.  transformer.  The  first  three  devices  are  ob- 
jectionable, in  that  the  voltage  used  is  too  low  to 
accurately  indicate  the  condition  of  the  insulation 
under  test ;  they  are  better  suited  to  prove  connec- 
tion between  parts  that  should  be  connected:  es- 
pecially is  the  bell  circuit  adapted  to  such  work 
as  "  ringing  out  "  car  connections.  Insulation 
testing  voltage  should  be  sufficient  to  send  a  signal 
current  through  existing  defects  and  to  break  down 
weak  spots  likely  to  break  down  later.  A  bridge 
or  bell  circuit  will  indicate  perfect  insulation  be- 
tween two  commutator  bars  that  almost  touch, 
because  the  resistance  of  the  thinnest  layer  of  air 
or  other  insulation,  so  far  as  low  voltage  will 
indicate,  is  perfect.  Any  test  using  500  volts  or 
more  would  indicate  the  insulation  to  be  below 
standard  and  would  probably  break  down  the 
weak  point  and  indicate  it  as  a  short-circuit. 

High  voltage  subjects  insulation  to  stresses  that 
weak  spots  cannot  stand;  but  if  too  high  and  in- 
judiciously applied,  insulation  formerly  perfect  for 
the  work  in  hand  may  be  injured.  Even  500-volt 
lamp  circuit  tests  can  do  harm  if  the  contacts  be- 
tween which  the  test  is  made  are  close  together, 


RESISTANCE  MEASUREMENTS.  49 

because  the  high  voltage  breaks  down  the  insula- 
tion and  starts  an  arc  that  carbonizes  surrounding 
surfaces.  Car  equipment  insulation  should  stand 
at  least  2000  volts — the  pressure  at  which  the 
lightning  arresters  act.  If  other  insulation  will 
allow  lightning  to  pass  more  readily  than  does  the 
arrester  path,  the  arrester  will  not  give  the  intended 
protection. 

VOLTMETER  METHOD. 

38.  The  measurement  of  insulation  with  a  volt- 
meter depends  on  the  fact  that  voltage  applied  to 


a  circuit  distributes  itself  according  to  the  distri- 
bution of  the  circuit  resistance.  In  Fig.  31,  T  is 
the  trolley  wire ;  V,  a  high-resistance  voltmeter,  and 
X,  a  sample  of  insulation  held  between  two  metal 
plates  that  do  not  touch  each  other.  On  closing 
K,  sufficiently  good  insulation  will  prevent  any  de- 
flection of  the  meter,  because  the  insulation  re- 
sistance is  so  high  compared  to  that  of  the  meter, 
that  the  entire  line  drop  is  across  the  insulation. 
This  does  not  mean  perfect  insulation,  for  a  higher 
voltage  or  more  sensitive  meter  would  show  it  to 


50  SHOP  TESTS. 

be  imperfect.  If  the  insulation  resistance  is  low 
enough  to  permit  an  appreciable  current,  there  will 
result  a  deflection  indicating  the  drop  taking 
place  through  the  voltmeter  resistance.  The  in- 
sulation here  acts  as  a  multiplier  of  unknown  re- 
sistance, the  value  of  which  is  to  be  determined 
experimentally  (see  Art.  12). 

39.  The  line  voltage  being  constant,  if  part  of 
it  drops  through  the  meter,  the  remainder  must 
drop  through  the  insulation,  so  that  the  difference 
between  the  line  voltage  and  meter  reading,  is  the 
insulation  drop.  The  voltmeter  and  insulation 
drops  will  have  the  same  relation  as  the  voltmeter 
and  insulation  resistances  that  cause  them.  Hav- 
ing the  two  drops  and  the  voltmeter  resistance, 
the  insulation  can  be  calculated  by  rule  20. 

Rule  20.  To  measure  insulation  with  a  volt- 
meter, connect  the  meter  and  insulation  in  series 
across  the  line.  Subtract  the  meter  defection  from 
the  line  voltage  to  get  the  insulation  drop;  then  mul- 
tiply the  meter  resistance  by  the  insulation  drop  and 
divide  by  the  voltmeter  drop. 

Example  34.  With  line  voltage  at  600,  the  p.d. 
in  an  80,000-ohm  voltmeter  in  series  with  the  test 
insulation,  is  500.  Wanted,  the  insulation  resist- 
ance. 

Solution  34.  As  line  voltage  is  600  and  meter 
reading  500,  the  insulation  drop  is  600  —  500  =  100 
volts.  By  rule  20,  then,  the  insulation  resistance  is 

(80,000  X 100)  -=-  500  =  8,000,000  •*•  500  =  16,000 
ohms.     Ans. 


RESISTANCE  MEASUREMENTS.  51 

40.  The  usual  rule,  given  as  rule  21,  uses  the 
insulation  drop  indirectly. 

Rule  21.  To  measure  insulation  resistance  with 
a  voltmeter,  connect  the  meter  and  insulation  in 
series  across  the  line,  the  insulation  having  a  switch 
K'  in  parallel  to  cut  it  in  and  out.  Take  a  reading 
with  the  insulation  cut  out  and  another  with  it 
cut  in.  Divide  the  reading  with  the  insulation  cut 
outf  by  that  with  the  insulation  cut  in;  then  subtract 
1  and  multiply  by  the  voltmeter  resistance. 


ARMATURE 


FLOOR' 

FIG.  32. 

Example  35.  With  an  80,000-ohm  voltmeter, 
the  reading  with  the  insulation  cut  out  is  600  and 
with  it  cut  in,  500.  Wanted  the  insulation  re- 
sistance. 

Solution  35.  By  rule  21,  600-7-500=1.2  and  1.2 
-  1  =  0.2  and  0.2  X  80,000  =  16,000  ohms  =  resistance 
of  the  insulation.  Ans. 

41.  The  method  of  making  the  test  on  a  ground- 
return  system  is  indicated  in  Fig.  32,  where  arma- 
ture insulation  from  commutator  to  shaft  is  to  be 
tested.  One  voltmeter  line  connects  to  trolley 


52  SHOP  TESTS. 

and  the  other,  t,  is  free.  If  the  armature  rests  on 
the  rail  or  other  grounded  part,  the  test  is  made 
as  follows:  touch  t  to  the  shaft;  as  the  shaft  is 
grounded,  the  needle  will  indicate  full  line  voltage, 
showing  the  test  circuit  to  be  in  order.  If  the 
armature  does  not  touch  a  grounded  part,  the 
shaft  or  core  must  be  grounded  by  a  second  line 
t',  as  in  Fig.  33.  In  either  case  the  next  step  is 
to  touch  line  t  to  the  commutator;  the  only  path 
for  test  current  to  reach  the  ground  is  through 


the  Insulation  between  the  copper  and  iron  parts 
of  the  armature  and  will  cause  a  deflection  indi- 
cating the  insulation  value. 

Example  36.  The  insulation  from  winding  to 
shaft  is  to  be  measured  on  an  armature  resting 
on  the  shop  floor.  The  voltmeter  is  connected  as 
in  Fig.  32,  the  armature  having  been  rolled  on  to 
the  track  rail.  On  touching  t  to  the  shaft,  the 
reading  is  500;  on  touching  it  to  the  commutator 
it  is  30.  What  is  the  armature  insulation  resist- 
ance? 


RESISTANCE  MEASUREMENTS.  53 

Solution  36.  Here  the  voltmeter  resistance  is 
80,000  ohms.  The  reading  with  the  insulation  cut 
out  is  500  and  with  the  insulation  cut  in  it  is  30. 
By  rule  21,  then,  the  insulation  resistance  is: 
500^-30=16.66  and  16.66-1  =  15.66  and  15.66  X 
80,000  =  1,253,000  ohms.  Ans. 

Note.  As  the  insulation  measures  over  1,000,000 
ohms  (called  1  megohm),  which  is  the  usual 
standard  of  insulation,  it  is  passed. 

Example  37.  Armature  insulation  is  to  be  meas- 
ured in  a  motor  installed  on  a  car.  How  can  it  be 
done  with  a  75,000-ohm  voltmeter? 

Solution  37.  Disconnect  the  armature  by  draw- 
ing its  brushes.  With  the  voltmeter  connected  as 
in  Fig.  32  or  33,  touch  /  to  the  motor  frame,  then 
to  the  commutator.  Assuming  the  first  deflection 
to  be  600  and  the  second  400  and  applying  rule  21, 
600 -^  400  =1.5  and  1.5-  1  =  0.5  and  0.5X75,000  = 
37,500  ohms.  Ans. 

Note.  On  cleaning  the  motor  with  an  air  blast 
and  scraping  and  wiping  the  end  of  the  commu- 
tator, the  insulation  deflection  was  reduced  to  40, 
corresponding  to  an  insulation  resistance  of  more 
than  a  megohm  and  showing  the  former  low  in- 
sulation resistance  to  have  been  due  almost  en- 
tirely to  accumulated  oil,  carbon,  dust  and  dirt. 

Example  38.  A  passenger  entering  a  car,  got  a 
shock  by  rubbing  against  the  rear  end  controller. 
How  can  the  insulation  from  the  controller  frame 
to  the  ground  be  measured  with  a  60,000-ohm, 
500-volt  voltmeter? 


54  SHOP  TESTS. 

Solution  38.  Connect  the  voltmeter  as  in  Fig.  32 
or  33.  Assume  that  touching  t  to  the  car  truck 
causes  a  deflection  of  490,  but  contact  with  the 
controller  frame  causes  a  deflection  of  only  45 
volts.  From  rule  21,  490-^45  =  10.9  and  10.9-1 
=  9.9  and  60,000X9.9  =  594,000  ohms,  resistance 
of  insulation.  Ans. 

Note.  Controller  frames  are  supposed  to  be  well 
grounded  so  that  an  internal  insulation  defect 
will  immediately  cause  a  demonstration  indicative 
of  that  fact.  With  a  defective  ground  connection 
defective  insulation  within  can  charge  the  frame 
which  can  then  shock  anyone  touching  it  and  a 
grounded  part  simultaneously.  "With  the  frame 
well  grounded,  touching  t  to  it  will  cause  full  line 
deflection.  The  preceding  test,  then,  indicates  a 
defective  frame  ground  connection  which  should 
be  immediately  repaired. 

42.  The  main  precaution  to  be  observed  in  lo- 
cating weak  insulation  with  a  voltmeter,  or  its 
equivalent,  is  to  disconnect  the  parts  liable  to 
defect  and  test  them  separately.  Such  a  test  will 
often  show  that  no  one  part  has  a  very  low  insu- 
lation resistance,  but  when  the  parts  are  connected 
as  in  operation,  thereby  paralleling  the  leakage 
paths,  the  total  insulation  resistance  may  be  com- 
paratively low.  To  illustrate:  If  a  voltmeter  on 
600  volts  be  used  to  measure  the  insulation  of 
each  unconnected  coil  on  a  newly  wound  arma- 
ture, no  single  coil  thus  tested  may  give  an  ap- 
preciable deflection ;  but  if  the  coil  leads  be  twisted 


RESISTANCE  MEASUREMENTS.  55 

together,  thereby  paralleling  the  leakage  paths, 
the  deflection  may  become  from  10  to  50  volts. 

Example  39.  Some  commutators  that  have  been 
taken  from  defective  armatures,  are  to  be  tested 
for  insulation  from  bar  to  bar  and  from  bars  to 
shell.  With  connections  of  Figs.  32-35,  how  can 
this  be  done? 

Solution  39.  Hold  one  test  line  on  the  shell  and 
successively  touch  every  bar;  then  touch  the  test 
lines  to  adjacent  bars,  doing  this  for  every  pair  of 
adjacent  bars;  in  every  case  note  the  voltmeter 


FIG.  34. 


SLOT  CONDUCTORS 
FIG.  35. 


deflection.  In  one  case  using  a  600- volt,  80,000-ohm 
voltmeter,  the  test  was  made  as  follows:  Line  /' 
was  held  to  the  shell  and  /  touched  to  each  bar 
to  see  that  none  was  actually  grounded  to  the 
shell ;  a  piece  of  small  copper  wire  was  then  twisted 
around  the  commutator,  to  connect  all  bars  and 
their  collective  insulation  to  shell  tested,  with 
the  bars  connected  together  the  deflection  was 
400  volts.  The  wire  was  then  removed  and  the  bar 
to  bar  insulation  tested;  in  every  case  the  bar  to 
bar  insulation  deflection  was  full  line  voltage  of 
550,  showing  the  insulation  between  bars  to  be  0. 


56  SHOP  TESTS. 

By  thoroughly  cleaning  the  commutator  ends, 
the  deflection  from  bars  to  shell  was  reduced  from 
400  to  40,  but  the  deflection  from  bar  to  bar  re- 
mained 550.  On  taking  a  3/32  inch  cut  off  the  com- 
mutator, the  bar  to  bar  deflection  was  reduced  to 
60  volts.  Baking  the  commutator  for  20  hours 
at  200  degrees  F.  reduced  the  bar  to  bar  reading 
to  4  and  the  bar  to  shell,  to  15  volts.  By  rule  21 
the  deflection  of  15  corresponds  to  an  insulation 
resistance  of  2,852,800  ohms  and  the  deflection 
of  4,  to  10  megohms. 

Notes.     Grease  and  paraffin  had  soaked  into  the 


FIG.  36. 

mica  bodies  and  heat  due  to  sparking  had  car- 
bonized them,  thereby  short-circuiting  the  commu- 
tator from  bar  to  bar — the  lathe  cut  removed  the 
carbonized  areas.  The  commutator  ends  were 
rilled  with  a  greasy  film  of  copper  and  carbon 
dust  that  bridged  the  insulation  from  bars  to 
shell; cleaning  removed  this.  The  final  baking  im- 
proved the  insulation  deflections  by  expelling  the 
accumulated  moisture. 

Figs.  34,  35  and  36  show  the  voltmeter  connec- 
tions, respectively,  on  a  double  overhead  trolley 
system,  slotted  conduit  system  and  third  rail  sys- 


RESISTANCE  MEASUREMENTS. 


57 


tern.  In  testing  from  a  slotted  conduit  system  it 
is  best  that  the  devices  under  test  make  no  contact 
with  the  ground.  When  this  is  unavoidable,  as 
when  testing  an  armature  in  a  motor,  care  must  be 
taken  when  connecting  test  line  t',  Fig.  35,  for  if 
one  of  the  conductor  rails  is  grounded  and  ground 
line  t'  is  run  to  the  conductor  rail  that  is  not 
grounded,  see  Fig.  37,  the  result  is  to  ground  both 
rails,  thereby  causing  a  short-circuit  likely  to  burn 


T-T+ 

rl 


T-   T* 


GROUNDED • 
>\      CONDUCTOR 
RAIL 

\ 


tf ^ 


WRONG    CONNECTION  RIGHT  CONNECTION 

FIG.  37. 


the  tester.  Where  two  test  lines  are  used  and  one 
conductor  rail  is  grounded,  run  the  ground  test 
line  to  the  grounded  rail.  With  one  conductor  rail 
grounded,  however,  but  one  test  line  is  needed,  as 
the  connections  of  Fig.  32  can  be  used. 

43.  In  voltmeter  insulation  tests  it  is  the  rule 
to  see  rather  that  the  insulation  reading  does  not 
exceed  a  specified  value,  than  to  find  the  actual 
insulation  resistance.  For  example,  if  the  man- 
agement decides  that  all  insulation  must  exceed  half 


68  SHOP  TESTS. 

a  megohm,  500,000  ohms,  it  is  then  necessary  to 
know  what  voltmeter  deflection  corresponds  to 
this  insulation  value. 

Rule  22.  To  find  the  deflection  corresponding 
to  a  specified  insulation  resistance,  with  a  volt- 
meter of  known  resistance,  on  a  line  of  known 
voltage,  divide  the  product  of  the  line  voltage  and 
the  voltmeter  resistance  by  the  sum  of  the  volt- 
meter resistance  and  the  specified  insulation  re- 
sistance. 

Example  40.  On  a  650-volt,  80,000-ohm  volt- 
meter, what  deflection  corresponds  to  an  insula- 
tion resistance  of  0.5  megohm,  the  line  voltage 
being  601? 

Solution  40.  By  rule  22,  601X80,000  =  48,080,- 
000,  and  500,000 +  80,000  =  580,000.  48,080,000- 
580,000  =  83  deflection.  Ans. 

Example  41.  To  what  deflection  would  a  resist- 
ance of  1  megohm  correspond? 

Solution  41.  By  rule  22,  601X80,000  =  48,080,- 
000  and  1,000,000  +  80,000  =  1,080,000.  48,080,000 
-=-1,080,000  =  46+  deflection.  Ans. 

LAMP  CIRCUIT  METHOD. 

44.  The  common  shop  and  road  method  for 
testing  insulation  is  with  a  lamp  circuit.  Fig.  38 
shows  connections  for  ground  return  and  Fig.  39, 
for  metallic  return  systems.  The  number  of  lamps 
to  be  connected  in  series  depends  on  the  voltages 
of  the  line  and  lamps  and  is  gotten  by  dividing  the 
line  voltage  by  the  voltage  marked  on  the  lamp 
bases,  all  lamps  being  the  same.  The  lamps  make 


RESISTANCE  MEASUREMENTS. 


59 


the  test  no  less  severe,  for  so  long  as  there  is  no 
current,  the  voltage  tending  to  puncture  the  insu- 
lation, is  the  total  line  voltage,  the  lamp  resistances 
being  negligible  compared  to  that  of  good  insula- 
tion; and  if  the  insulation  fails,  the  lamps  prevent 
violent  demonstrations.  The  tests  are  made  the 
same  as  the  voltmeter  tests;  if  the  lamps  light  up, 
the  insulation  is  poor  or  zero,  according  to  the 
brilliancy  of  the  lamp. 


FIG.  38.  FIG.  39. 

Insulation  that  is  below  standard  may  be  too 
good  to  admit  sufficient  current  to  give  any  lamp 
indication;  if  on  tapping  the  test  point,  a  little 
arcing  is  observed,  the  insulation  is  not  what  it 
should  be.  In  practice  the  correct  number  of 
lamps  in  series  are  mounted  on  a  paddle  and  pro- 
vided with  test  lines,  as  in  Fig.  40,  and  a  cleat 
provided  on  which  to  wind  them  when  not  in  use. 

45.  When  the  source  of  voltage  is  a  ground  return 


GO  SHOP  TESTS. 

system  and  neither  of  the  contacts  between  which 
insulation  is  to  be  tested  touches  ground,  a  ground 
test  line,  t',  Fig.  33,  must  be  run,  otherwise  the 
test  circuit  will  not  be  complete.  Where  one  con- 
tact is  grounded,  but  one  test  line,  t,  is  required, 
but  sometimes  two  are  provided  to  cover  all  con- 
ditions. When  using  the  test  lines,  one  of  which  is 
grounded,  to  test  insulation  between  contacts,  one 
of  which  is  grounded,  the  trolley  test  line,  t,  must 
be  applied  to  the  ungrounded  contact  and  grounded 
test  line,  t',  to  the  grounded  contact.  Without  this 
precaution,  the  tester  is  liable  to  touch  the  trolley 


FIG.  40. 

test-line  to  a  grounded  part  and  get  an  indication 
of  short-circuit  when  none  exists.  Thus  in  Fig.  33 
the  armature  does  not  touch  the  rail,  but  should 
such  a  contact  be  made  unbeknown  to  the  tester, 
if,  in  applying  the  test-lines,  he  happened  to  touch 
t  to  the  shaft,  the  lamps  \vould  light  independently 
of  the  existence  of  t',  although  the  tester  might 
not  know  it. 

46.  The  lamp  test  is  sometimes  used  to  test 
the  insulation  from  bar  to  bar  on  a  commutator 
just  machined,  the  current  being  sufficient  to  burn 
out  short-circuits  due  to  filings,  chips  and  burrs; 


RESISTANCE  MEASUREMENTS.  61 

the  test  may  do  more  harm  than  good,  for  burning 
out  a  "  short  "  starts  an  arc  that  may  char  sur- 
rounding insulation  and  make  it  worse  than  be- 
fore the  test.  On  this  account  it  is  well  to  use  low 
voltage  and  apply  it  intermittently  by  tapping 
the  test  point  on  the  bar  to  make  and  break  con- 
tact. For  burning  out  commutator  shorts,  a.c. 
voltage  is  better  as  it  tends  less  to  hold  an  arc. 

BELL  CIRCUIT  METHOD. 

47.  The  low  voltage  of  a  bell  circuit  is  of  little 
use  in  testing  the  condition  of  insulation  between 


conductors  that  should  not  touch.  Such  tests  will 
indicate  an  actual  short-circuit,  but  they  are  more 
used  to  test  the  continuity  of  conductors  and  to 
identify  the  ends  of  concealed  wires,  such  as  car 
wires.  A  bell  circuit  usually  consists  of  a  wooden 
box  with  a  handle  and  cleats  for  coiling  the  test 
lines  when  not  in  use  (see  Fig.  41).  In  the  box 
are  four  dry  cells  in  series  and  on  top  is  an  electric 
bell.  The  battery,  bell  and  test  lines  are  connected 
in  series,  so  that  touching  the  test  points  together 
closes  the  circuit,  thereby  ringing  the  bell.  In 
testing,  hold  the  points  together  before  and  after 


62  SHOP  TESTS. 

each  test  to  see,  by  the  ringing  of  the  bell,  that 
the  circuit  is  in  order.  In  applying  the  test  points 
to  opposite  ends  of  the  same  conductor,  the  bell 
will  ring  if  the  conductor  is  continuous.  Bell  cir- 
cuits are  not  adapted  to  test  for  open  circuits  in 
high-resistance  conductors,  such  as  fine  wire  mag- 
net coils,  the  current  being  insufficient  to  ring  the 
bell  even  when  no  break  exists. 

MAGNETO  METHOD. 

48.  A  magneto,  such  as  that  used  for  calling  on 
a  telephone,  is  an  a.c.  dynamo;  turning  the  crank 
rotates  an  armature  in  a  field  produced  by  per- 
manent magnets.  Two  test  lines,  /,  t',  connect  to 
binding  posts  that  represent  the  armature  ter- 
minals. As  with  the  bell  test,  the  test-lines  are 
touched  before  and  after  each  test  to  insure  con- 
tinuity, and  tests  are  made  similarly,  the  test 
points  being  applied  to  the  contacts  between 
which  the  test  for  insulation  or  continuity  is  to 
be  made.  A  magneto  belted  to  shop  shafting  can 
be  made  to  give  from  200  to  300  volts,  in  which 
case  it  will  give  good  results  in  testing  insulation. 
At  times  (just  frequent  enough  to  be  puzzling)  a 
magneto  will  give  misleading  indications,  because 
its  current  is  alternating  and  the  contacts  between 
which  insulation  is  being  tested,  may  have  great 
capacity  like  a  condenser:  the  bell  will  vibrate  al- 
though the  insulation  may  be  practically  perfect. 
The  effect  can  best  be  seen  by  holding  the  test 
points  to  the  terminals  of  a  condenser  and  turning 


RESISTANCE  MEASUREMENTS.  63 

the  crank.  If  the  condenser  capacity  is  sufficient 
the  bell  will  ring  owing  to  the  charging  and  dis- 
charging current  that  is  produced  with  each  alter- 
nation. Some  car  wiring  systems  and  large  gen- 
erator armatures  are  able  to  produce  this  mis- 
leading effect. 


HIGH  VOLTAGE  INSULATION  TEST. 

49.  Such  tests  are  usually  a.c.  at  voltages  of 
1,000  or  more.  The  manufacturing  companies  pro- 
vide suitable  apparatus  for  this  work,  but  a 
homemade  outfit  can  be  made  at  small  cost.  Fig. 
42  illustrates  diagrammatically  a  high-tension  out- 
fit that  did  good  service  for  years.  A  is  an  old 
two-pole  railway  motor  connected  to  the  line 
through  switch  K.  On  the  pinion  end  of  the 
armature  are  two  insulated  metal  rings;  one  taps 


F.IELO 


to  the  back  end  of  the  conductor  connecting  to 
any  commutator  bar  and  the  other  to  the  back  end 
of  the  conductor  connecting  to  the  opposite  bar. 
On  closing  K  the  motor  runs  as  a  d.c.  series  motor, 
as  it  would  on  a  car,  but  brushes  bearing  on  the 
rings  will  deliver  alternating  current.  500  volts 
on  the  d.c.  end  give  about  300  volts  on  the  a.c. 
end,  the  frequency  being  the  number  of  revolu- 
tions per  second  of  the  armature.  As  the  a.c. 
voltage  lights  three  110- volt  lamps  in  series  to 
64 


HIGH  VOLTAGE  INSULATION  TEST.         65 

normal  brilliancy,  its  value  is  about  330  volts. 
This  a.c.  e.m.f.  is  applied  to  the  primary  coil  of  a 
homemade  transformer  Tl,  the  secondary  coil  5 
which  has  six  times  as  many  turns  as  the  primary 
p,  so,  theoretically,  the  a.c.  test  voltage  would  be 
6  X  330  =  1,980  volts.  The  secondary  voltage  lights 
to  full  candle-power,  eighteen  100-volt,  16  c-p. 
lamps  L,  L  when  test  lines  t,  t',  are  held  together; 
the  a.c.  test  voltage  available  is,  then,  1,800  volts. 
The  manner  of  applying  the  high- voltage  test  lines 
is  theoretically  the  same  as  that  for  low-voltage 
lines;  practically,  owing  to  the  greater  danger,  ex- 
treme care  must  be  taken  to  prevent  personal  con- 
tact with  high-tension  parts.  The  test  points  should 
be  held  on  the  ends  of  dry  maple  poles  5  ft.  long; 
the  testing  space  should  be  enclosed  and  but  one 
man  allowed  within  the  enclosure  when  the  motor 
is  running. 

To  test  the  winding-to-shaft  insulation  of  an 
armature,  for  example:  Hook  one  test  point  over 
the  armature  shaft;  close  Kl,  then  K,  to  start  the 
test  motor;  pick  up  the  second  test  point  at  the 
far  end  of  its  pole,  touch  test  points  /  and  t'  to- 
gether to  see  that  the  lamps  light;  then  hold  t  to 
the  commutator  for  10  seconds;  if  the  insulation 
is  standard,  the  lamps  will  not  light  with  it  in  cir- 
cuit; but  if  defective,  the  high  voltage  will  punc- 
ture it  and  lighting  of  the  lamps  will  indicate  that 
fact.  Having  made  the  test,  hang  up  test  point  t, 
stop  the  motor,  open  Kl  and  remove  point  /'. 

Too  much  stress  cannot  be  laid  on  the  import- 


66  SHOP  TESTS. 

ance  of  avoiding  contact  with  any  high-tension 
part;  what  might  be  trivial  on  a  500- volt  circuit 
might  here  cause  death.  As  the  test  is  not  used 
continuously,  the  transformer  need  not  be  made 
with  the  customary  restrictions  in  regard  to  tem- 
perature and  efficiency.  An  old  ring  armature  can 
be  converted  to  transformer  use  by  dividing  the 
winding  into  two  parts  such  that  one  part  has  six 
times  as  many  turns  as  the  other;  use  the  smaller 
section  as  the  primary  and  the  larger  as  the  sec- 
ondary of  the  test  transformer.  With  a  little  care 
the  outfit  can  be  mounted  to  make  a  good  ap- 
pearance. 

Note.  In  modern  high-tension  testing  sets,  in- 
struments are  used  to  indicate  the  presence  of  in- 
sulation faults  and  the  alternating  current  from 
the  same  source  is  applied  to  indicating  the  pres- 
ence of  short-circuits  in  windings.  These  applica- 
tions will  be  considered  in  detail  in  a  pamphlet  on 
Railway  Equipment  Tests. 


MISCELLANEOUS  TESTS. 

ARMATURE  TESTS. 

BAR-TO-BAR  TESTS. 

50.  The  object  of  a  bar-to-bar  test  is  to  sec  if 
all  coils  of  an  armature  have  approximately  the 
same  resistance;  decided  variation  in  its  resistance 
means  irregularity  in  the  coil.     It  may  have  too 
many  or  too  few  turns,  the  wrong  size  of  wire,  a 
poor  connection,  an  open  or  a  short-circuit  or  the 
armature  may  be  roasted  in  spots.   To  get  the  best 
results,  the  instruments  must  be  low  reading  so 
that  a  small  change  in  resistance  will  cause  a  large 
change  in  deflection;  the  voltage  should  be  con- 
stant to  avoid  changes  in  deflection  due  to  changes 
in  voltage.   Knowledge  of  the  absolute  resistance  of 
the  coils  under  test  is  an  advantage,  as  variations 
in  deflection  can  be  easily  checked.     For  absolute 
measurements,  the  instruments  must  be  correct. 
Shop  tests  are  usually  limited  to  comparing  the 
deflections  of  a  voltmeter  as  its  terminals  are  ap- 
plied to  adjacent  bars  all  round  the  commutator, 
the  current  being  maintained  constant. 

51.  Directions.     Connect  the  ammeter,  a  vari- 
able resistance  and  the  armature  in  series,  as  in 
Fig.  43,  the  current  being  applied  to  the  armature 
at  the  points  usually  occupied  by  brushes  in  op- 
eration— a  brush  holder  and  brushes  can  be  used 
to  advantage.     Hold  the  voltmeter  terminals  on 

67 


68  SHOP  TESTS. 

adjacent  bars  and  adjust  the  current  until  the 
voltmeter  gives  a  readable  deflection.  Then  con- 
tinuing to  hold  the  test  points  in  this  same  rela- 
tion, slowly  turn  the  armature  and  note  the  suc- 
cessive deflections  as  the  volt  lines  make  contact 
with  the  successive  pairs  of  bars.  Keep  the  current 
as  constant  as  possible. 

52.  Remarks.  In  case  of  discrepancy  in  read- 
ings, they  must  be  repeated,  extra,  care  being 
taken  to  keep  the  current  constant.  A  high  reading 
may  mean  too  small  a  wire  in  the  coil,  too  many 


AM. 


turns  or  a  poor  connection;  a  low  reading  may 
mean  too  large  a  wire,  too  few  turns  or  a  short- 
circuit  in  the  coil  or  between  the  bars,  to  which 
the  coil  is  connected.  The  cause  of  the  discrepancy 
must  be  found  by  inspection. 

53.  An  open-circuited  coil  will  cause  all  readings 
except  three  to  be  twice  what  they  should  be  for 
a  given  current,  because  the  open-circuit  leaves 
but  one  path  through  the  armature,  thereby  doub- 
ling its  resistance;  as  the  tester  keeps  the  current 
at  the  same  value  as  if  the  two  halves  of  the  arma- 
ture were  in  parallel,  the  drop  in  each  coil  is  doubled 


MISCELLANEOUS  TESTS.  69 

because  its  normal  current  has  been  doubled.  Two 
of  the  remaining  deflections  will  be  normal,  corre- 
sponding to  the  two  positions  where  the  brushes 
make  contact  with  the  two  bars  that  include  the 
open-circuit  and  cut  it  out;  the  coil  then  spanned 
by  the  test  points  gets  half  the  total  current,  as  it 
should,  and  the  deflection  is  normal.  The  third 
odd  deflection  is  away  too  high,  because  the  test 
points  span  the  bars  that  include  the  open-circuit; 
the  voltmeter  is  then  subjected  to  the  drop  in 
half  the  coils  in  the  armature,  the  coils  on  both 
sides  of  the  test  points  acting  as  volt  lines  to  con- 
nect the  test-points  to  the  brushes  where  the  path 
of  the  current  enters  and  leaves  the  good  half  of 
the  armature. 

54.  If  made  of  sufficient  capacity  so  as  not  to 
foam,  a  water  rheostat  can  be  used  for  current 
regulation;  as  the  water  becomes  heated  and  its 
resistance   reduced,   its  effective  length   must  be 
increased  to  keep  the  current  constant.    Where  a 
storage  battery  or  other  source  of  constant  voltage 
is  used,  no  ammeter  is  required  unless  absolute 
resistances  are  wanted.     If  the  test  is  prolonged, 
heating  will  increase  the  drop  per  coil,  but  as  the 
change  is  small  and  gradual,  it  need  not  be  mis- 
leading.    It  is  desirable,  on  this  account,  to  use 
the  least  current  that  will  give  a  readable  deflec- 
tion and  also  the  current  must  be  kept  within  the 
maximum  rating  of  the  armature. 

55.  Current  to  Give  a  Certain  Deflection.     The 
instruments   must  not  be   subjected   to   currents 


70  SHOP  TESTS. 

exceeding  their  maximum  rating.  It  is  easy  to 
keep  within  the  range  of  an  ammeter  because  its 
maximum  graduation  indicates  its  maximum  rat- 
ing and  the  variable  resistance  required  can  be 
calculated  by  rule  16,  the  line  voltage  being  known. 
To  keep  within  the  maximum  reading  of  a  milli- 
voltmeter,  however,  an  approximate  idea  of  the 
resistance  of  the  armature  coils  under  test  must 
be  had;  then  the  current  that  will  give  a  drop 
within  the  limit  of  the  meter  can  be  calculated  by 
rule  9. 

Example  42.  In  an  armature  bar  to  bar  test,  the 
maximum  range  of  the  ammeter  available  is  50 
amperes.  Neglecting  the  resistance  of  the  arma- 
ture, what  extra  resistance  will  keep  the  current 
under  50  amperes,  the  line  voltage  being  500? 

Solution  42.  By  rule  16, 

500  ^  50  =  10  ohms.     Ans. 

Example  43.  A  millivoltmeter  reading  of  450 
millivolts  (0.450  volts)  is  to  be  used  in  a  bar-to-bar 
test  on  an  armature,  each  coil  of  which  measures 
0.012  ohm.  What  is  the  maximum  current  that 
can  be  used? 

Solution  43.  Here  the  resistance  on  which  the 
meter  reads  the  drop  is  0.012  ohm  and  the  current 
through  it  must  be  such  that  the  drop  may  not 
exceed  0.450  volts.  This  current  is,  then,  by  rule  9, 
0.450 -T- .012  =  37.5  amp.  As  37.5  amperes  is  the 
maximum  allowable  current  per  coil,  and  as  each 
coil  gets  but  half  the  total  current,  the  total  cur- 


MISCELLANEOUS  TESTS.  71 

rent — that  for  which  the  variable  resistance  would 
be  calculated — would  be  2X37.5  amperes  =  75  am- 
peres. Ans. 

Notes.  This  current  would  produce  a  drop 
within  the  limits  of  the  voltmeter,  but  would  ex- 
ceed the  rating  of  the  armature  in  question.  If 
there  is  no  idea  as  to  the  resistance  of  an  armature 
coil  and  no  measuring  means  are  available,  it  can 
be  approximated  by  rinding  size  and  length  of 
wire  in  a  coil;  multiply  the  length  in  feet  by  the 
resistance  per  foot  as  giver  in  a  wire  table.  Modern 
armatures  may  include  2  or  more  coils  in  a  single 
slot;  so  care  must  be  taken,  when  measuring,  to 
measure  only  what  will  be  included  between  two 
commutator  bars  after  connecting  the  armature. 

56.  Current  within  the  Armature  Rating.  Given 
the  horse  power  of  an  armature,  the  maximum 
allowable  testing  current  can  be  calculated  by 
rule  23. 

Rule  23.  To  get  the  maximum  test  current  to 
be  used  on  an  armature  of  given  horse  power  and 
voltage,  multiply  the  horse  power  rating  by  746  and 
divide  by  the  rated  voltage.  The  result  will  be  full 
load  current. 

Example  44.  A  bar-to-bar  test  is  to  be  made 
on  a  G.  E.  1,000  armature.  Assuming  the  rating 
to  be  35  hp.  at  500  volts,  find  the  full  load  current. 

Solution  44.  By  rule  23, 

35X746  =  26,110  and  26, 1 10 -f- 500  =  52  amperes. 
Ans. 

Example  45.  The  rating  of  a  Westinghouse  12  A 


72  SHOP  TESTS. 

motor  being  25  hp.  at  500  volts,  what  would  be 
the  maximum  current  to  use  in  a  bar-to-bar  test? 

Solution  45.  25  X  746  =  18 ,650  and  18,650  ^  500  = 
37.3  amperes.  Ans. 

67.  Where  the  horse  power  of  a  motor  is  not  ob- 
tainable, the  maximum  current  to  be  used  in  test- 
ing can  be  calculated  approximately  by  rule  24: 

Rule  24.  To  get  the  approximate  safe  testing 
current  for  a  motor  of  unknown  horse  power,  find 
the  size  of  wire  in  the  coils  and  get  its  cross  section 
from  a  wire  table ;  divide  the  cross  section  in  circular 
mils  by  450;  the  result  will  be  the  safe  current  for 
each  wire. 

Note.  If  the  coil  is  wround  with  single  wire, 
multiply  the  safe  current  per  wire  by  2;  if  wound 
with  two  wires  in  parallel,  multiply  by  4. 

Example  46.  A  G.  E.  1,000  armature  is  wound 
with  single  No.  9  B.  &  S.  D.  CC.  copper  wire.  What 
is  the  safe  testing  current  value? 

Solution  46.  Cross  section  No.  9=13,000  cir- 
cular mils  (round  numbers)  and  13,000-^-450  =  29 
and  29X2  =  58  amperes.  Ans. 

Note.  The  correct  full-load  current  by  rule  23, 
is  52  amperes;  as  the  test  current  is  made  less 
than  maximum  permissible  current,  to  avoid  heat, 
this  result  is  sufficiently  close.  On  more  modern 
armatures,  the  results  of  applying  rule  24  will  be 
more  correct. 

Example  47.  A  Westinghouse  No.  68  armature 
is  wound  with  No.  12  B.  &  S.  wire;  as  each  com- 
mutator ear  has  four  leads  in  it,  either  the  coils 


MISCELLANEOUS  TESTS.  73 

are  wound  two  wires  in  parallel  or  the  coils  are 
connected  two  in  parallel.  In  either  case  the  cur- 
rent path  from  brush  to  brush  consists  of  four  No. 
12  wires  in  parallel.  What  would  be  a  safe  test 
current  ? 

Solution  47.  Cross  section  No.  12  =  6,500  cir. 
mils  (round  numbers)  and  6,500-7-450  =  14.5  and 
14.5X4  =  58  amperes.  Ans. 

Note.  The  No.  68  is  rated  as  a  40-hp.  motor 
and  rule  23  would  give  60  amperes  as  full  load 
current  at  500  volts. 


FIG.  44. 

LEAD-TO-BAR  TEST. 

58.  Imperfect  connections  between  leads  and 
commutator  are  sometimes  indicated  by  an  eaten 
out  appearance  of  parts  of  the  commutator.  The 
connections  can  be  tested  as  indicated  in  Fig.  43, 
only  instead  of  applying  the  test  points  to  ad- 
jacent bars,  one  point  is  touched  to  a  bar  and  the 
other  to  its  connecting  lead,  as  in  Fig.  44.  A  de- 
flection that  is  high  as  compared  to  the  general 
run  of  deflections  indicates  an  imperfect  con- 
nection. 


74 


SHOP  TESTS. 


TELEPHONE  TESTS. 

69.  Short-Circuit  Test.  A  simple  quick  test  for 
open  and  short-circuits  in  the  armature  can  be 
made  with  a  telephone  receiver  connected  as  in 
Fig.  45.  Current  is  introduced  into  the  armature 
as  in  Figs.  43,  44  and  45. 

The  source  of  current  may  be  a  couple  of  dry 
cells,  a  d.c.  or  a.c.  lighting  circuit.  Where  direct 
current  is  used,  either  one  of  the  test  points  must 
be  tapped  intermittently  to  one  of  the  bars 


FIG.  45. 

touched  or  an  electric  bell  or  buzzer  must  be  in- 
cluded in  the  supply  circuit,  because  unless  the 
current  is  made  to  vary,  the  telephone  diaphragm 
will  be  attracted  once  when  the  circuit  is  closed 
and  will  remain  in  that  position  until  the  circuit 
is  opened;  to  give  its  characteristic  sounds  a  tele- 
phone must  be  acted  on  by  a  varying  current  that 
alternately  allows  the  receiver  diaphragm  to  be 
attracted  and  released.  Where  an  alternating  cur- 
rent is  available,  no  current  interrupter  is  required. 
60.  With  the  test  points  applied  to  adjacent 


MISCELLANEOUS  TESTS.  75 

bars,  the  telephone  receiver  coil  may  be  subjected 
to  two  influences:  first,  there  is  a  p.d.  in  each 
armature  coil,  due  to  its  resistance,  as  the  current 
varies,  this  p.d.  will  also  vary  and  thereby  im- 
press on  the  telephone  coil  the  variable  voltage 
required  to  produce  the  vibration  of  the  diaphragm. 
The  intensity  of  the  vibration,  hence -of  the  sound 
given  by  the  telephone,  will  depend  on  the  p.d. 
to  which  the  telephone  coil  is  subjected  and  this 
on  the  resistance  of  the  armature  coil.  Assuming 
an  armature  coil  of  normal  resistance  to  give  a 
telephone  sound  of  normal  intensity,  an  armature 
coil  of  greater  or  less  resistance  will  give  a  sound 
of  greater  or  less  intensity;  thus  a  short-circuited 
armature  coil  would  produce  little  or  no  sound 
because  it  would  have  little  or  no  resistance, 
hence  little  or  no  p.d.  across  it;  an  open-circuited 
armature  coil  would  give  a  sound  of  disagreeable 
intensity,  because  the  telephone  would  be  sub- 
jected to  the  p.d.  in  the  coils  of  half  the  armature; 
between  these  limits  the  intensity  of  sound  depends 
on  whether  the  fault  increases  or  decreases  the 
resistance  of  the  armature  coil  and  how  much. 
If  a  direct  current  is  used  without  an  interrupter 
the  action  described  is  the  only  one  that  occurs 
and  it  would  be  possible  to  pass  without  detec- 
tion, a  coil  of  too  few  turns  of  too  small  a  wire 
because  of  its  resistance  being  just  right.  Second, 
where  the  supply  is  a.c.  or  d.c.  with  interrupter, 
a  second  effect,  called  the  auto-transformer  effect, 
obtains. 


76  SHOP  TESTS. 

An  alternating  or  pulsating  current  through  the 
armature  magnetizes  and  demagnetizes  the  core, 
thereby  setting  in  motion,  magnetic  lines  of  force 
that  cut  the  armature  coils  and  generate  in  them 
voltages  opposed  to  the  supply  voltage.  An  ex- 
planation of  the  transformer  effects  would  be  out 
of  place  here  but  it  may  be  said  that  the  telephone 
coil  spanning  the  armature  coil  under  test,  creates 
a  local  circuit  through  which  the  back- voltage  due 
to  the  moving  lines  of  force  can  act  to  establish 
a  secondary  current  that  affects  the  telephone; 
any  fault,  such  as  too  many  turns  in  the  armature 
coil,  increases  the  current  in  the  local  circuit  and 
the  intensity  of  the  sound;  any  fault  such  as  too 
few  turns  in  the  armature  coil,  decreases  the  cur- 
rent in  the  local  circuit  and  decreases  the  intensity 
of  the  sound.  The  difference  in  sound  due  to  dif- 
ference in  the  transformer  action  is  sufficient  to 
detect  a  coil  of  the  correct  resistance  that  may 
have  too  many  or  too  few  turns.  As  the  two 
effects  conspire  to  the  same  end,  they  cannot  be 
separated  nor  can  they  confuse.  The  duty  of  the 
tester  is  simply  to  try  every  armature  coil  and  in- 
vestigate any  that  give  abnormal  intensity  of 
sound.  The  alternating  current  test  is  however 
to  be  preferred  to  the  test  with  the  direct  current. 

61.  Ground  Test.  Another  useful  application  of 
the  telephone  is  to  locate  the  faulty  bar  or  coil  in 
a  grounded  armature,  a  ground  on  a  ground-return 
circuit  being  a  special  case  of  short-circuit.  A 
battery  or  d.c.  lamp  circuit  with  interrupter  or  an 


MISCELLANEOUS  TESTS. 


77 


a.c.  lamp  circuit  without  any  interrupter,  is  ap- 
plied as  in  Fig.  46.  Here  5  is  the  armature  shaft 
and  one  of  the  armature  coils  is  supposed  to  be 
grounded  to  it  or  the  core  through  a  fault  at  a. 
To  make  the  test,  one  terminal  of  the  'phone  is 
held  to  the  shaft  and  the  other  touched  around 
the  commutator  from  bar  to  bar.  Ordinarily  the 
telephone  will  click  because  it  is  subjected  to  a 
p.d.  represented  by  the  drop  in  the  armature  coils 
that  extend  from  the  fault  to  the  bar  touched  by 
the  free  test  line  t;  however,  as  soon  as  the  free 


test  line  touches  the  bar  to  which  the  faulty  coil 
connects,  the  click  ceases  or  becomes  a  minimum, 
but  increases  in  intensity  if  t  is  moved  on  beyond 
the  faulty  connection.  When  t  is  touched  to  the 
faulty  bar,  both  test  lines  are  connected  to  the 
shaft;  both  ends  of  the  telephone  being  then  at 
the  same  potential,  it  is  subjected  to  no  p.d., 
takes  no  current,  hence  emits  no  noise. 

DIFFERENTIAL  VOLTMETER  TEST. 
62.  Where  the  test  voltage  is  variable,  a  low 
reading  differential  voltmeter  is  well  adapted  to 


78  SHOP  TESTS. 

indicate  differences  in  armature  resistances.  Fig. 
47  indicates  the  connections.  Current  is  passed  as 
in  the  bar-to-bar  test;  one  coil  of  the  differential 
voltmeter  is  permanently  connected  to  one  of  the 
armature  coils  or  to  a  standard  coil  of  the  same 
resistance  so  connected,  as  to  get  the  same  current 
hence  the  same  drop.  The  other  voltmeter  coil  is 
connected  to  test  points  that  are  moved  from  bar 
to  bar.  Assuming  the  resistances  of  all  armature 
coils  to  be  the  same  as  that  of  the  standard,  on 
passing  the  test  lines  from  bar  to  bar  the  volt- 


G  =. 
FIG.  47. 

meter  needle  will  stay  at  0,  because  the  equal  drops 
oppose  each  other  in  their  action  on  the  needle. 
If  the  resistance  of  a  coil  is  above  or  below  normal, 
however,  the  drop  in  it  will  be  more  or  less  than 
that  in  the  standard,  the  current  in  both  being 
the  same,  and  the  excess  will  move  the  needle  to 
one  side  or  the  other  of  0  and  by  an  amount  equal 
to  the  voltage  by  which  the  drop  on  one  coil  ex- 
ceeds that  on  the  other.  If  the  test  coil  resistance 
exceeds  that  of  the  standard,  the  needle  will  deflect 
to  the  right,  say;  but  if  the  standard  resistance 


MISCELLANEOUS  TESTS.  79 

exceeds  that  of  the  test  coil,  the  needle  will  deflect 
to  the  left.  The  cause  of  any  marked  discrepancy 
must  be  located  by  inspection. 

TRANSFORMER  TEST. 

63.  A  modern  method  of  testing  finished,  con- 
nected armatures  for  short-circuits,  is  with  an  a.c. 
transformer  connected  as  in  Fig.  48.  Here  A  is  a 
50-hp.  railway  motor  pole  piece,  slotted  on  the 
concave  side  so  as  to  take  about  200  turns  of  No. 
10  B.  &  S.  magnet  wire.  Angle  iron  plates  a  and  b 


ARMATURE  - 

FIG.  48. 

support  the  wire  in  winding  and  protect  it  in  op- 
eration. The  winding  has  terminals  and  a  small 
switch  K  mounted  on  the  shoe  so  that  the  test 
current  can  be  conveniently  interrupted.  The 
shoe  is  supported  on  legs  so  that  it  can  be  leaned 
against  the  armature,  or  has  a  bail  with  which  to 
hang  it  on  to  a  hoist  so  that  it  can  be  raised  or 
lowered.  In  any  case  the  shoe  is  made  to  engage 
the  core  of  the  armature  to  be  tested;  the  a.c. 
lines  are  then  connected  to  the  binding-posts  and 
K  closed.  The  shoe  and  coil  and  the  armature  core 


80  SHOP  TESTS. 

normally  act  as  a  choke  coil  and  the  shoe  winding 
takes  but  little  current.  Each  alternation  of  the 
current  causes  magnetic  lines  of  force  to  move 
and  cut  all  armature  conductors  lying  between 
the  test  shoe  horns.  As  each  slot  holds  conductors 
belonging  to  both  halves  of  the  armature  and  as 
both  halves  have  the  same  number  of  turns  and 
as  the  current  in  them  is  in  opposite  directions, 
so  long  as  the  armature  has  no  short-circuited  coil, 
the  tendency  of  the  magnetic  lines  to  produce 
current  in  one  half  of  the  winding,  is  balanced  by 
the  equal  but  opposite  tendency  to  produce  cur- 
rent in  the  other  half,  the  result  being  zero  current. 
A  short-circuit  in  a  coil  or  cross  between  the  bars 
to  which  it  connects,  creates  a  local  circuit  in  one 
half  of  the  winding  but  not  in  the  other,  thereby 
destroying  the  balance;  so  that  the  moving  lines 
of  force  cause  a  current  as  soon  as  any  conductor 
involved  in  the  fault  lies  between  the  two  test 
shoe  horns.  On  a  railway  motor  armature  of  the 
usual  type,  a  short-circuit  will  affect  four  armature 
conductors  because  each  pair  of  bars  contains 
leads  from  two  coils  the  legs  of  which  lie  in  four 
equidistant  slots  on  the  core.  Any  demonstration 
at  a  given  slot  1  will  be  manifest  at  the  almost 
diametrically  opposite  slot  3  and  at  slots  2  and  4, 
Fig.  48,  half  way  between  1  and  3.  Current  in 
the  local  circuit  established  by  the  fault  sends  out 
lines  of  force  that  react  on  the  lines  due  to  the 
test  shoe  coil ;  the  result  of  this  reaction  is  to  weaken 
the  magnetism  of  the  shoe,  decrease  its  choking 


MISCELLANEOUS  TESTS.  81 

effect  and  admit  a  larger  current.  One  method  of 
indicating  a  short-circuit  is  to  place  in  series  with 
the  shoe  coil  an  ammeter  the  reading  of  which  will 
be  above  normal  when  a  short-circuit  exists.  As 
the  tester  soon  becomes  familiar  with  the  current 
taken  under  normal  conditions,  the  ammeter  read- 
ing indicates  at  once  the  existence  of  abnormal 
conditions. 

A  shoe  the  horns  of  which  are  sufficiently  far 
apart  to  send  the  magnetic  lines  through  the  core 
in  the  same  relation  as  would  the  pole  pieces  in 
which  the  armature  operates,  gives  the  widest 
range  of  effect,  but  such  a  shoe  is  comparatively 
heavy. 

64.  The  more  usual  method  of  indicating  a 
short-circuit  is  with  a  thin  iron  strip  held  in  the 
hand  and  passed  around  the  core  as  follows:  Turn 
on  the  current  and  hold  the  vibrator  to  the  core 
at  points  about  at  right  angles  to  the  point  midway 
between  the  shoe  horns;  then  stop  the  current, 
rotate  the  armature  a  little,  start  the  current 
again  and  again  apply  the  vibrator;  repeat  these 
operations  until  all  slots  have  been  tested.  The 
instant  a  conductor  involved  in  the  fault  comes 
under  the  vibrator,  the  vibrator  vibrates  in  a 
characteristic  manner. 

Note.  When  there  is  alternating-current  in  the 
local  circuit  due  to  the  fault,  the  coils  composing 
the  local  circuit  produce  an  alternating  field  which 
causes  the  iron  strip  to  vibrate  whenever  it  is  over 
a  slot  containing  a  conductor  involved  in  the  local 
circuit. 


82  SHOP  TESTS. 

LATHE  TEST. 

65.  Where  an  alternating  current  is  not  avail- 
able for  a  transformer  test,  an  equivalent  test  can 
be  made  with  a  test  shoe  excited  by  direct  current. 
In  this  case  it  is  best  that  the  shoe   horns   span 
the  core  as  do  the  poles  of  the  motor  in  which  the 
armature  operates.     The  armature  is  swung  in  a 
lathe  and  the  test  shoe  run  up  near  to  it;  provision 
being  made  to  prevent  the  magnetism  from  draw- 
ing the  shoe  actually  against  the  core;  the  shoe  is 
excited,  the  lathe  started  and  the  vibrator  held  near 
the  core.     A  short-circuit,  howsoever  slight,  will 
cause    the    vibrator   to    act   in    an    unmistakable 
manner.    The  magnet  can  be  wound  with  fine  wire 
and  the  current  held  to  a  safe  value  by  incan- 
descent lamps  in  series,  or  it  can  be  wound  with 
coarse  wire  and  a  heavy  current  used  and  regulated 
with  a  water  rheostat. 

SPINNING  TEST. 

66.  The   spinning   test   consists   in  running  the 
motor  at  full  line  voltage  before  or  after  it  has 
been  hung  on  the  truck.    This  test  shows  the  con- 
dition of  the  bearings  and  shaft,  the  set  of  the 
brushes,  open  and  short-circuits  and  the  direction 
of  rotation  for  given  connections.     For  this  test, 
connect  one  field  terminal  through  resistance  to 
trolley,  connect  one  of  the  brushholders  to  ground; 
connect  the  remaining  field  terminal  and  brush- 
holder  together.    Unless  the  motor  frame  rests  on 
a  grounded  part,   a  ground  connection  must  be 


MISCELLANEOUS  TESTS.  83 

run  from  the  frame  to  the  track  rail  or  one  brush- 
holder  must  be  connected  to  the  motor  frame, 
otherwise  the  presence  of  a  fault-ground  will  cause 
no  indication. 

67.  Bearings  and  Shaft.     The  armature  bearing 
caps  are  tightened  before  the  spin;  if  the  bearings 
heat  they  are  removed  and  scraped.      A  sprung 
shaft  will  wobble  on  the  end.    An  unbalanced  core 
will  cause  excessive  vibration  but  the  shaft  ends 
will  not  wobble.      An  eccentric  commutator  will 
cause  the  brushes  to  draw  in  and  out  of  the  holders 
as  the  armature  rotates.    A  high  or  low  bar  or  flat 
commutator  will  make  the  brushes  chatter.     End 
play  is  tested  by  pushing  on  both  ends  of  the  shaft 
and  noting  the  travel:  the  end  play  should  not 
exceed  £  inch. 

68.  Brush  Set.     Radial  brushes  should  rest  flat 
on  the  commutator;  on  brush  holders  of  the  inde- 
pendent type  an  even  bearing  across  the  brush 
contact  indicates  correct  brush  set.     On  holders 
of  the  yoke  type,  however,  the  motor  should  be 
loaded  with  a  brake  to  see  that  the  brushes  do 
not  spark.     In  either  case,  if  one  or  both  brushes 
bear  on  the  heel  or  toe,  they  need  adjustment. 
The  distance  between  the  inside  edges  of  the  brushes 
should  be  one  quarter  of  the  commutator  circum- 
ference less  the  width  of  one  brush. 

69.  Open  and  Short  Circuit.    An  open  circuit  in 
the  field  winding  will  prevent  starting.    An  arma- 
ture open-circuit  will  cause  a  rotating  spark.    An 
armature   short-circuit   will   cause   jerky   rotation 


R4 


SHOP  TESTS. 


and  a  strip  of  iron  held  near  the  head  of  the  ar- 
mature will  pulsate  in  a  characteristic  manner. 

70.  Grounds.  To  insure  that  the  test  shall  in- 
dicate field  as  well  as  armature  grounds,  the  field 
winding  must  be  connected  next  to  the  trolley  as 
in  Fig.  49.  With  the  armature  winding  next  to 
the  trolley,  its  counter  e.m.f.  consumes  most  of 
the  line  voltage,  leaving  insufficient  e.m.f.  to  cause 
a  demonstration  even  if  a  field  coil  is  grounded. 
Failure  to  observe  this  precaution  will  allow  a 


FIG.  49. 

grounded  field  coil  to  pass  the  spinning  test  with- 
out being  detected. 

71.  Connections.  By  observing  a  fixed  rule  in 
connecting  the  motors,  then  noting  the  direction 
of  rotation  in  each  case,  the  tester  can  detect  ir- 
regularities in  rotation;  thus,  in  the  case  of  a  so- 
called  left-hand  armature  or  in  case  the  field  coils 
as  a  whole  are  connected  in  the  wrong  relation  to 
the  armature  winding,  rotation  will  be  opposite 
to  the  usual  direction.  If  the  coils  are  regular  in 
winding  and  connection,  the  armature  must  be 


MISCELLANEOUS  TESTS.  85 

irregular.  In  all  modern  controllers,  the  +  arma- 
ture connections  are  marked  with  A  and  the 
—  armature  connections  with  A  A ,.  and  the  +  field 
connections  with  F  and  the  —  field  connections  with 
E,  a  number  placed  after  the  letters  indicating  to 
which  motor  the  connections  belong.  Thus  Al  is 
the  +  armature  connection  of  motor  No.  1  and 
E4,  the  —  field  connection  of  motor  No.  4,  and  so 
on.  If  the  spinning  te°,t  shows  that  the  motor 
armature  turns  clockwise  when  viewed  from  the 
commutator  end  and  the  left  hand  brushholder 
and  top  field  terminals  are  made  +,  then  when 
the  motor  is  on  a  car  it  follows  that  to  have  the 
armature  turn  clockwise  the  left  hand  brushholder 
must  be  made  A  and  the  top  field  terminal,  F, 
the  remaining  brushholder  being  A  A  and  the 
bottom  field  terminal,  E.  Knowing  the  direction 
of  rotation  of  the  motor  for  given  connections,  the 
manner  of  connecting  the  motor  terminals  to  the 
tagged  car-wires  is  fixed  irrespective  of  the  po- 
sition of  the  motor  on  the  truck.  As  the  armature 
and  car  wheels  are  geared  together,  the  direction 
of  movement  of  the  car  will  be  opposite  to  that 
of  the  top  of  the  armature. 

FIELD  CONNECTION  TEST. 

72.  Adjacent  field  coils  in  a  motor  should  pro- 
duce poles  of  opposite  polarity;  a  nail  can  be  used 
to  find  out  if  they  do  or  not.  Open  the  motor,  re- 
move the  armature,  close  the  motor  and  establish 
a  safe  current  through  the  field  coils  connected  in 


SHOP  TESTS. 


series.  Holding  the  nail  between  the  thumb  and 
forefinger,  present  the  point  to  any  pole  piece  and 
move  the  nail  toward  either  adjacent  pole  piece; 
if  the  coils  are  correctly  connected,  the  nail  will 
persevere  in  the  position  of  T,  Fig.  50,  and  effort 
will  be  required  to  make  it  take  any  other  general 
position  in  its  passage  from  one  pole  to  the  next 
one.  This  is  because  it  lies  in  the  general  direction 
of  the  magnetic  lines  of  force  running  from  the  N 
to  the  5  poles,  and  is  unstable  in  any  other  posi- 
tion. Also  on  touching  the  nail  to  the  pole  pieces 


FIG.  50. 


FIG.  51. 


FIG.  52. 


and  pulling  it  away,  the  pull  on  all  will  be  about 
the  same.  Fig.  51  shows  the  general  path  of  the 
magnetic  lines  when  one  field  coil  is  connected 
wrongly,  making  three  S  poles  and  one  N  pole; 
here  on  either  side  of  the  wrongly  connected  coil 
no  effort  will  be  required  to  turn  the  nail  to  po- 
sition Tl,  the  nail  seeming  to  have  no  well  directed 
tendency.  Also  the  pole  due  to  the  wrongly  con- 
nected coil  will  exert  less  pull  than  the  others. 
Fig.  52  shows  the  stable  positions  of  the  nail  when 
two  coils  are  wrongly  connected.  On  every  pole, 
the  nail  will  act  normally  on  one  side  but  will 


MISCELLANEOUS  TESTS.  87 

have  no  directed  tendency  on  the  other;  also  one 
horn  of  each  pole  piece  will  be  much  stronger  than 
the  other.  In  this  case,  to  right  matters,  reverse 
the  connections  of  the  coils  on  the  two  pole  pieces 
between  which  the  nail  action  is  normal.  Where  a 
motor  has  but  two  field  coils,  as  in  the  case  of  the 
G.  E.  800,  no  test  is  needed,  because  if  one  coil 
is  reversed  the  armature  is  unable  to  start  without 
excessive  current. 

With  practice  the  nail  test  can  be  made  without 
removing  the  armature.  To  a  beginner  the  poles 
induced  in  the  armature  core  by  the  pole  pieces  is 
apt  to  be  confusing.  This  simple  test  is  applicable 
to  any  kind  of  machine  of  any  number  of  poles. 

LOCATING  CHARGED  AND  GROUNDED  CAR 

PARTS. 

73.  Car  parts  normally  alive  when  the  motor 
circuit  is  active  may  become  "  grounded  "  and 
cause  irregular  action;  similarly,  parts  normally 
grounded  may  become  dangerous  as  the  result  of 
becoming  "  charged."  Thus  the  starting  coil  re- 
sistance metal  is  alive  in  operation  but  is  supposed 
to  be  thoroughly  insulated  from  the  ground.  A 
"  dead  ground  "  on  the  starting  coil  will  prevent 
a  car  from  starting  as  the  current  short-circuits 
to  ground  without  reaching  the  motors;  a  partial 
ground,  however,  is  even  more  to  be  avoided  as 
it  may  result  in  charging  some  part  on  which  a 
passenger  must  step.  Controller  frames  are  well 
grounded  so  that  they  may  not  become  charged 


88  SHOP  TESTS. 

from  defective  internal  insulation  and  shock  a 
person  touching  them  and  grounded  parts  at  the 
same  time. 

74.  A  charged  or  grounded  condition  can  be 
detected  and  its  cause  exactly  located  with  a  volt- 
meter. To  test  for  a  grounded  part,  connect  one 
side  of  the  voltmeter  to  trolley  and  use  a  free  test 
line  from  the  other  side  to  explore  by  contact, 
the  suspected  area  and  its  connections  after  having 
disconnected  all  intentional  ground  connections  to 
that  area.  The  instant  the  test  point  touches  a 
grounded  part,  the  voltmeter  needle  will  deflect, 
because  current  from  the  trolley  has  its  path 
through  the  meter  to  and  through  the  fault,  to 
ground.  To  test  for  charged  parts,  connect  one 
side  of  the  voltmeter  to  ground  and  explore  the 
suspected  area  with  a  free  test  line  from  the  other 
side  of  the  meter.  Contact  with  a  live  part  will 
cause  a  deflection,  because  the  live  part  becomes 
grounded  through  the  voltmeter. 

Rule  25.  To  test  for  a  ground  with  a  voltmeter, 
connect  one  end  of  the  meter  to  trolley  and  ex- 
plore with  the  other.  To  test  for  live  parts  with  a 
voltmeter,  ground  one  side  of  the  meter  and  ex- 
plore with  the  other.  Where  several  connected 
devices  cause  a  deflection,  disconnect  them  and 
test  each  separately. 

Example  48.  On  a  certain  car  in  damp  weather 
passengers  stepping  on  to  the  car  platform  from 
inside,  get  a  shock.  Find  a  cause  with  a  voltmeter. 

Solution  48.  On  grounding  one  side  of  a  500-volt 


MISCELLANEOUS  TESTS.  89 

voltmeter  and  exploring  with  the  other,  contact 
with  a  resistance  hanger  bolt  extending  through 
the  car  floor  gave  a  deflection  of  475  volts.  Inspec- 
tion showed  defective  insulation  between  the  re- 
sistance metal  and  frame,  so  that  as  soon  as  the 
controller  was  put  on  an  operating  notch,  thereby 
charging  the  resistance  metal,  the  frame,  hanger 
and  supporting  bolts  became  charged  and  a  person 
stepping  immediately  over  the  bolt  head  and  on 
to  the  damp  platform  simultaneously,  formed  a 
circuit  through  which  the  electric  shock  could  pass. 

Note.  Had  the  voltmeter  shown  full  line  de- 
flection it  would  have  meant  a  "  dead  ground  " 
and  the  car  could  not  have  been  started. 

Example  49.  On  a  certain  car  the  conductor  got 
a  shock  every  time  he  "  rang  up  "  a  fare.  How 
could  the  cause  of  this  condition  be  located? 

Solution  49.  As  the  register  rigging  was  confined 
to  the  upper  parts  and  ends  inside  of  the  car,  the 
bulkhead  wiring  was  suspected.  With  the  pole  on 
and  both  breakers  closed,  one  side  of  a  voltmeter 
was  grounded  and  the  other  side  touched  to  the 
register  rod;  full  line  deflection  indicated  actual 
contact  with  a  trolley  part.  On  disconnecting  the 
register  rod  and  lifting  the  register  from  its  hang- 
ers, connected  only  when  the  register  was  in  place, 
only  one  of  the  hangers  gave  appreciable  deflec- 
tion; removal  of  one  of  the  hanger  screws  did 
away  with  all  deflection.  Inspection  showed  the 
screw  to  have  penetrated  the  insulation  of  the 
bulkhead  trolley-wire,  thereby  charging  the  regis- 
ter pulls  through  the  rigging. 


90  SHOP  TESTS. 

Example  50.  On  a  certain  stove  heated  car 
equipped  with  independent  air  brake,  contact  with 
the  stove  caused  a  shock  even  on  a  dry  day.  Why? 

Solution  50.  On  grounding  one  side  of  the  volt- 
meter and  touching  the  other  side  to  the  stove, 
there  was  no  deflection,  showing  the  stove  to  be 
free  from  charge.  On  connecting  one  side  of  the 
meter  to  trolley  and  the  other  to  the  stove,  full 
line  deflection  showed  the  stove  to  be  well  grounded. 
As  the  stove  was  the  ground  end  of  the  trouble 
circuit  the  "  live  "  end  had  to  be  sought  elsewhere. 
On  grounding  one  side  of  the  meter  and  exploring 
the  area  around  the  stove,  a  charged  bolt  head 
was  found.  The  bolt  passed  through  a  piece  of  tin 
under  the  car  and  though  no  longer  used  was  in 
contact  with  a  hanger  of  the  governor  pipe.  On 
this  equipment  the  compressor  and  all  of  the  gov- 
ernor parts  were  insulated  from  all  ground  parts 
with  an  insulation  joint,  to  lessen  the  chances  of 
grounds.  One  of  the  compressor  armature  terminals 
was  grounded  to  the  frame,  thereby  charging  the 
frame,  governor  pipe,  hanger,  tin,  bolt  and  the  area 
around  the  bolt  head,  so  that  a  person  standing 
on  the  bolt  head  and  touching  the  stove  was  di- 
rectly across  the  500-volt  circuit. 

Example  51.  Another  car  gave  the  same  symp- 
toms only  the  shock  was  less  severe. 

Solution  51.  On  grounding  one  side  of  the  volt- 
meter and  touching  the  other  side  to  the  stove, 
full  line  deflection  showed  the  stove  to  be  charged 
An  uninsulated  two-way  connector  that  con- 


MISCELLANEOUS  TESTS.  91 

nected  the  governor  to  trolley  lay  on  an  angle  iron 
that  fastened  the  stove  base  to  the  floor.  The 
stove  happened  not  to  be  grounded. 

Example  52.  On  a  certain  flat  car,  the  trolley 
wire  from  the  trolley  base  to  the  circuit  breaker 
was  run  in  an  iron  pipe.  A  person  standing  on 
the  cement  floor  and  touching  the  pipe,  got  a 
shock.  Why? 

Solution  52.  Had  the  pipe  been  grounded  it 
would  have  given  no  shock;  on  grounding  one  side 
of  the  voltmeter  and  touching  the  other  side  to 
the  pipe,  the  deflection  was  450  on  a  line  voltage 
of  500.  On  disconnecting  the  trunk  wire  from  the 
base  and  testing  insulation  from  wire  to  pipe  (see 
Art.  40)  no  deflection  obtained;  on  reconnecting 
the  trunk  wire  to  the  base  and  testing  the  insula- 
tion of  wire  to  pipe,  a  deflection  of  450  again  ob- 
tained, indicating  the  defect  to  be  in  the  base. 
Next  the  pipe  was  pulled  loose  from  the  base  and 
the  insulation  from  wire  to  pipe  tested;  it  was 
perfect.  On  replacing  the  pipe,  grounding  it 
through  a  lamp  circuit,  connecting  one  side  of  the 
voltmeter  to  trolley  and  exploring  the  stretch  of 
wood  between  the  base  and  the  pipe  any  deflection 
from  0  to  425  could  be  gotten.  This  showed  that 
the  defective  insulation  was  the  oak  board  on 
which  the  base  was  mounted  and  to  which  the 
pipe  was  cleated.  A  maple  board  did  away  with 
all  leakage. 

Note.  Oak  contains  acid  and  cannot  be  de- 
pended on  as  an  insulator. 


92  SHOP  TESTS. 

GENERAL  TESTING  PRECAUTIONS. 

75.  Correct  results  require  either  that  the  in- 
struments be  correct  or  that  their  error  be  exactly 
known;  the  condition  of  an  instrument  can  be 
determined  by  comparing  its  readings  with  those 
of  another  known  to  be  correct.  If  the  condition 
of  an  instrument  is  unknown,  make  the  test,  check 
the  instrument  with  a  standard  afterward  and 
make  necessary  corrections  in  the  calculations.  To 
avoid  error  in  reading,  be  certain  that  the  needle 
is  just  over  its  own  reflection  in  the  mirror. 

Portable  instruments  should  not  be  put  in  their 
boxes  upside-down;  in  the  box  the  needle  should 
hang  down.  In  use  it  is  generally  best  that  the 
instrument  rest  level.  In  presence  of  vibration, 
as  on  a  car,  hold  the  instruments  in  a  box  partially 
filled  with  waste. 

Never  subject  an  ammeter  to  excessive  current 
nor  a  voltmeter  to  excessive  voltage;  in  all  cases 
get  an  approximate  idea  of  the  maximum  value 
to  be  indicated  and  if  excessive,  use  a  shunt  or  a 
multiplier  as  the  case  may  be.  Always  connect  the 
+  side  of  an  instrument  to  the  +  side  of  the  cir- 
cuit. The  +  side  of  the  instrument  is  generally 
indicated  by  a  -f-  mark  and  the  +  side  of  the  cir- 
cuit can  be  ascertained  by  test  or  inquiry.  Where 
a  meter  has  extra  connecting  posts  unfamiliar  to 
the  tester,  consult  someone  who  knows  about  them ; 
the  method  of  trial  may  here  cause  trouble. 

In  testing  with  an  ammeter  or  voltmeter  or 
both,  use  instruments  the  rating  of  which  is 


MISCELLANEOUS  TESTS.  93 

adapted  to  the  quantities  to  be  indicated;  to  illus- 
trate, don't  use  a  500- volt  voltmeter  to  read  a 
drop  of  3  volts,  nor  a  150-amp.  ammeter  to  indi- 
cate the  current  of  a  lamp  circuit,  because  since 
each  division  has  a  high  value  an  error  in  reading 
will  have  a  correspondingly  high  value;  on  the 
other  hand  don't  connect  a  millivoltmeter  across 
a  high  drop  nor  use  a  milliammeter  to  read  a  large 
current.  Never  try  to  use  an  ammeter  as  a  volt- 
meter as  it  will  certainly  cause  trouble  for  op- 
erator and  instrument. 

On  variable  voltage,  repeat  readings  and  sets 
of  readings  until  it  is  certain  that  they  are  correct ; 
especially  valuable  are  repeated  readings  where 
simultaneous  ammeter  and  voltmeter  readings  are 
to  be  taken.  In  comparing  resistances  care  must 
be  taken  that  they  are  at  the  same  temperature, 
because  the  resistance  of  the  common  conductors, 
except  carbon  and  water,  increase  with  tempera- 
ture. In  such  tests  use  the  minimum  allowable 
current  and  dispose  the  resistances  so  that  they 
will  heat  at  equal  rates.  To  check  results  it  is 
well  to  repeat  whole  tests  at  different  current  and 
voltage  values  and  then  compare  the  results  of 
calculation. 

Never  close  a  switch  unless  certain  of  existing 
conditions;  but  once  having  decided  to  close  it, 
close  it  with  a  will  so  that  should  conditions  be 
such  as  to  produce  a  short-circuit,  the  tester  will 
not  be  burned.  Always  include  in  circuit,  a  fuse, 
circuit  breaker  or  other  safety  device,  to  protect 
the  ammeter  in  case  of  short-circuit. 


94  SHOP  TESTS. 

HELP  TO  THE  INJURED. 
REVIVING  SHOCKED  PERSONS. 
76.  The   following   directions  for  reviving  per- 
sons from  the  effects  of  electric  shock  (or  apparent 
drowning)  are  due  in  substance  to  Augustin  Goelet, 
M.D.,  and  are  adapted  from  the  Electrical  World 
and  Engineer  Supplement  of  September   6,  1902. 
In  all  cases  the-  operations  described  are  to  be 
begun  without  delay  and  continued  until  the  arrival 
of  a  physician. 

I.  Remove  the  body  from  the  live  conductor. 
If  in  midair,  poke  it  loose  with  a  wooden  pole 
and  catch  it  in  a  blanket  held  at  the  four  corners, 
unless    there    are    present    facilities    and    persons 
qualified  to  safely  use  more  refined  methods.    If 
on  the  surface  use  a  dry  stick  or  protect  the  hands 
with  dry  clothing. 

II.  Turn  the  body  upon  the  back,  loosen  the 
clothing  around  the  neck  and  chest  and  place  a 
rolled-up  coat  under  the  shoulders  to  throw  the 
head  back  and  mouth  open.    Kneeling  at  the  vic- 
tim's head,  seize  both  arms  and  draw  them  to 
full  length  and  almost  together  over  the  head,  as 
in  Fig.  53,  to  expand  the  chest  and  open  the  wind- 
pipe; hold  this  position  for  two  or  three  seconds; 
next  carry  the  arms  down  to  the  sides,  Fig.  54, 
showing  the  half-way  position,  and  front  of  the 
chest,  firmly  compressing  the  chest  walls,  as  indi- 
cated in  Fig.  55,  to  expel  the  air  from  the  lungs. 
These  successive  operations  of  drawing  the  arms 
back   over  the   head   almost   together,   and   then 


MISCELLANEOUS  TESTS.  95 


FIG.  54. 


FIG.  55. 


96  SHOP  TESTS. 

bending  them  as  in  Fig.  54,  and  finally  compressing 
them  on  the  chest  side  walls,  as  in  Fig.  55,  must 
be  repeated  from  sixteen  to  eighteen  times  per 
minute  and  continued  ceaselessly  for  at  least  an 
hour,  or  until  breathing  is  normal.  (This  method 
has  been  known  to  resuscitate  patients  who  had 
been  under  water  several  hours.) 

III.  While  artificial  breathing  is  being  thus  con- 
ducted a  second  person  should  grasp  the  victim's 
tongue    with   a    handkerchief    (forcing   the    teeth 
apart  with  a  knife  or  piece  of  wood  if  necessary 
and  pull  the  tongue  out  in  step  with  the  stretching 
back  of  tne  arms,  and  allowing  it  to  recede  into 
the  mouth  when  the  chest  is  compressed. 

IV.  Dashing  cold  water  in  the  face,  brisk  rub- 
bing of  the  spine  with  ice,  or  alternate  heating 
and  cooling  of  the  region  over  the  heart,  all  tend 
to  produce  a  gasp  and  thereby  start  breathing, 
which  should  then  be  continued  artificially  until 
it  becomes  natural.    It  is  both  useless  and  unwise 
to  try  to  revive  the  patient  by  pouring  stimulants 
down  the  throat.    In  all  cases  send  for  a  physician. 

RELIEVING  BURNS. 

77.  The  simplest  and  most  satisfactory  relief 
for  an  electric  burn  is  to  immerse  the  effected  part 
in  a  mixture  of  linseed  oil  and  soda,  and  to  keep 
it  there  until  all  soreness  is  removed.  In  dangerous 
localities,  where  numbers  of  men  are  employed,  a 
barrel  of  this  mixture  should  be  kept  on  hand.  In 
case  of  severe  body  burns  the  patient  may  stand 


MISCELLANEOUS  TESTS.  97 

in  the  barrel.     In  all  cases  it  is  best  to  consult  a 
physician. 

QUESTIONS 

1.  How   is   an   ammeter  connected    to   indicate 
current  in  a  conductor? 

2.  Does  it  make  a  difference  which  connecting 
post  is  made  +  ? 

3.  Does  the  ammeter  resistance  appreciably  af- 
fect the  test  current? 

4.  Can  large  currents  be  indicated  on  two  am- 
meters in  parallel? 

5.  What  is  requisite  for  two  meters  to  read  their 
joint  rating? 

6.  In  what  two  ways  can  the  sharing  of  current 
between  them  be  regulated? 

7.  Why  are  these  devices  objectionable  for  per- 
manent work? 

8.  What   is   meant   by   shunting   an   ammeter? 
What  is  the  constant? 

9.  How  can  a  shunt  be  adjusted  with  a  second 
ammeter? 

10.  How  can   a  shunt  be   adjusted   without  a 
second  ammeter? 

11.  What  is  the  usual  objection  to  a  fractional 
constant  ? 

12.  When  is  a  fractional  constant  permissible? 

13.  If  the  shunt  heats,  will  the  constant  change? 

14.  Will  a  shunt  adjusted  for  one  meter  do  for 
another? 

15.  Give  the  rule  for  determining  the  constant 
of  a  certain  adjustment? 


98  SHOP  TESTS. 

13.  If  the  shunt  heats,  will  the  constant  change? 

14.  Will  a  shunt  adjusted  for  one  meter  do  for 
another? 

15.  Give  the  rule  for  determining  the  constant 
of  a  given  adjustment? 

16.  Can  current  be  measured  with  a  voltmeter 
and  known  resistance? 

17.  Give  the   connections  and  the   rule  to  be 
applied. 

18.  How  can  the  current  taken  by  a  car  be  ap- 
proximately ascertained? 

19.  Give  the  rule  for  measuring  current  with  a 
wattmeter. 

20.  On  what  two  factors  do  wattmeter  indica- 
tions depend? 

21.  If  volts  be  multiplied  by  amperes,  what  unit 
results? 

22.  If  watts  be  divided  by  volts,  what  unit  re- 
sults? 

23.  If  watts  be  divided  by  amperes,  what  unit 
results  ? 

24.  Watt-hour   meter   records   are   usually   ex- 
pressed in  what  units? 

25.  How  can  the  watt  rate  be  gotten  from  the 
watt-hour  record? 

26.  Does  a  watt-hour  meter  give  information  as 
to  volts  and  amperes? 

27.  How  can  average  volts  or  amperes  during 
the  time  of  a  record  be  gotten? 

28.  How  is  voltage  or  potential  drop  measured 
with  a  voltmeter? 


MISCELLANEOUS  TESTS.  99 

32.  What  would  be  the  objection  to  a  low  re- 
sistance voltmeter? 

33.  Why  does  a  voltmeter  in  series  with  other 
resistances   take    almost    the    entire    line    voltage 
across  its  own  terminals? 

34.  What  is  the  effect  of  connecting  a  resistance 
in  parallel  with  another? 

35.  Why  does  not  a  voltmeter  do  this  to  an  ob- 
jectionable degree? 

36.  Can  high  voltages  be  indicated  on  two  volt- 
meters in  series? 

37.  Will   two   meters   so   connected   necessarily 
read  the  sum  of  their  ratings?    What  condition  is 
required  for  them  to  do  so? 

38.  In  connecting  two  voltmeters  in  series,  how 
are  the  posts  connected? 

39.  How  can  the  maximum  voltage  readable  on 
two  meters  in  series  be  determined?  What  is  meant 
by  a  voltmeter  multiplier! 

40.  What  is  meant  by  the  constant  of  the  volt- 
meter and  multiplier? 

41.  How  can  the  constant  of  a  given  pair  be 
calculated  ? 

42.  How  can  it  be  determined  experimentally? 

43.  Give  connections  for  experimental  determi- 
nation of  the  constant. 

44.  About  what  is  the  resistance  of  a  600-volt 
voltmeter?     500- volt  voltmeter? 

45.  How  can  the  multiplier  resistance  required 
to  increase  the  range  of  a  given  voltmeter  a  certain 
amount  be  calculated? 


100  'SHOP  TESTS, 

46.  What  is  the  approximate  resistance  of  a  16- 
c-p.,  110- volt  lamp? 

47.  Can  incandescent  lamps  in  series  be  used  as 
a  multiplier? 

48.  Can  p.d.  be  measured  with  an  ammeter  and 
a  known  resistance? 

49.  Give  the  connections  and  the  rule  to  be  ap- 
plied to  do  so. 

50.  Is  it  generally  practicable  to  vary  the  volt- 
age of  a  supply  dynamo? 

51.  What  is  the  common  method  of  lowering 
the  voltage  acting  on  a  device? 

52.  Howr  does  voltage  applied  to  a  circuit  dis- 
tribute itself? 

53.  Can  storage  cells  be  used  to  vary  test  volt- 
ages? 

54.  Does  the  voltage  of  a  storage  battery  depend 
on  the  number  of  cells  in  series  ? 

55.  WThy  are  storage  cells  not  more  used  in  regu- 
lating test  voltages? 

56.  What  is  meant  by  proportion  lines!    Sketch 
their  connections. 

57.  What  is  the  advantage  of  having  the  resist- 
ance across  the  line  composed  of  similar  units? 
Name  a  good  practical  form  of  resistance  unit. 

58.  How  can  the  voltage  acting  on  the  meter 
be  then  calculated? 

59.  Can  the   voltage  be   first  explored  with  a 
voltmeter? 

60.  In  what  field  are  proportion  lines  especially 
useful? 


MISCELLANEOUS  TESTS.  101 

61.  Can  proportion  lines  be  used  for  adjusting 
a  voltmeter  and  multiplier  from  a  voltage  exceed- 
ing the  maximum  rating  of  the  voltmeter? 

62.  Will  the  adjustment  thus  made  on  low  volt- 
age do  for  all  voltages? 

63.  Can    voltages   be    measured    approximately 
with  incandescent  lamps  in  series?    Give  the  con- 
nections and  method  of  making  them. 

64.  How  is  the  total  voltage  acting  calculated 
from  the  lamp  indication? 

65.  Is  the  method  applicable  to  alternating-cur- 
rent (a.c.)  circuits? 

66.  Will  low-voltage  lamps  give  closer  results 
than  high- voltage  lamps? 

67.  What  is  meant  by  resistance  as  applied  to 
electric  circuits? 

68.  Do   insulators  offer  great   resistance?      Do 
conductors? 

69.  Does  conductor  resistance  depend  on  length? 
Size?     Material? 

70.  When  are  conductors  said  to  be  connected 
in  series'? 

71.  When  are  conductors  said  to  be  connected  in 
parallel? 

72.  When  are  conductors  said  to  be  connected 
in  series-parallel? 

73.  How  may  the  resistance  of  conductors  in 
series  be  calculated? 

74.  Is  the  resistance  of  two  conductors  in  par- 
allel less  than  that  of  either  alone  ?    What  is  meant 
by  parallel  resistance  of  conductors? 


102  SHOP  TESTS. 

75.  What   is   the    approximate    resistance    of   a 
32-c-p.,  110- volt  lamp? 

76.  What  would  be  the  resistance  of  five  such 
lamps  in  series? 

77.  Are  the  lamps  on  a  single  car-lamp  circuit 
in  series  or  in  parallel? 

78.  Give  rule  for  calculating  parallel  resistance 
of  two  like  conductors. 

79.  Give  rule  for  calculating  parallel  resistance 
of  any  two  conductors. 

80.  Give  rule  for  calculating  resistance  of  any 
number  of  like   or  unlike   conductors  in   parallel 
and  calculate  for  1,  2,  3  and  4  ohms? 

81  Give  rule  for  calculating  the  resistance  to 
be  connected  in  parallel  with  a  certain  conductor 
to  make  their  parallel  resistance  of  a  certain  value. 

82.  What  resistance  must  be  connected  in  par- 
allel with  a  conductor  measuring  10  ohms  for  their 
parallel  resistance  to  be  7  ohms? 

83.  Give  and  apply  the  rule  for  calculating  the 
resistance  to  be  connected  in  parallel  with  a  num- 
ber of  existing  parallel  conductors  to  make  their 
final  parallel  resistance  have  a  certain  value. 

84.  Is  this  rule  useful  in  designing  parallel  start- 
ing coils  for  cars? 

85.  What   advantage   has  such  a  starting  coil 
over  the  usual  series  type? 

86.  Can  resistances  be  measured  with  a  volt- 
meter and  ammeter? 

87.  Give  the  connections  and  rule  to  be  applied. 

88.  Give  the  approximate  resistance  used  on  a 
20-ton  electric  car. 


MISCELLANEOUS  TESTS.  103 

89.  Give  connections  and  method  for  measuring 
heater  resistances. 

90.  Can  resistance  be  measured  with  voltmeter 
and  known  resistance? 

91.  Give  connections  and  rules  to  be  applied  in 
this  measurement. 

92.  Why  is  it  important  to  repeat  the  voltmeter 
readings? 

93.  How  can  the  resistance  of  railway  motors 
field  coils  be  tested. 

94.  Will  this  test  reveal  the  condition  of  the 
field  coils? 

95.  What  indication  suggests  that  the  coils  be 
opened  and  inspected? 

96.  In  testing  field  coils  is  it  customary  to  work 
out  resistances? 

97.  How  can  the  approximate  resistance  of  wire 
be  gotten  from  a  wire  table? 

98.  Does  the   condition   of  the   car  equipment 
affect  its  resistance? 

99.  Does  resistance  affect  the  line  voltage  lost 
in  the  equipment? 

100.  Can  resistance  be  measured  with  an  am- 
meter and  a  known  resistance? 

101.  Is  this  a  method  of  great  accuracy?    If  not, 
why  not? 

102.  Give  the  connections  and  the  rule  to  be 
applied  in  the  test. 

103.  Why  is  it  important  to  get  both  ammeter 
readings  at  same  voltage? 

104.  Why  are  simultaneous  readings  on  a  railway 
circuit  hard  to  get? 


104  SHOP  TESTS. 

105.  Give    the    main    construction    points    of   a 
differential  voltmeter. 

106.  Do  variations  in  line  voltage  affect  both 
deflecting  coils? 

107.  Must  the  pairs  of  posts  be  so  connected  as 
to  oppose  each  other? 

108.  Give  the  method  of  measuring  starting  coils 
differentially. 

109.  About  what  should  be  the  rating  of  the 
meter  for  this  work? 

110.  Is  it  necessary  to  keep  the  current  below 
a  certain  value?     Why? 

111.  At  about  what  value  is  the  current  regu- 
lated?    By  what  means? 

112.  Describe  a  test  on  a  car  resistance  coil  in 
place. 

113.  Why  is  the  car  brake  applied? 

114.  What  would  be  the  advantage  of  a  scale 
under  the  standard  resistance? 

1 15.  Why  is  the  standard  resistance  made  greater 
than  that  on  the  cars? 

116.  How  is  a  phig  bridge  distinguished  from  a 
slide  wire  bridge? 

117.  Why  are  plug  bridges  not  adapted  to  shop 
work? 

118.  Describe  a  homemade  slide  wire  bridge. 

119.  Why  should  the  battery  key  be  pressed 
before  the  galvanometer  key? 

120.  What  portion  of  the  wire  gives  the  most 
accurate  balance  point? 

121.  WThen  is  a  bridge  said  to  be  balanced? 


MISCELLANEOUS  TESTS.  105 

122.  How  is  the  balance  effected?     Can  a  tele- 
phone be  used  instead  of  a  galvanometer;  in  such 
cases  why  musl  the  stylus  be  tapped  on  the  wire? 

123.  Where  the  exact  balance  point  is  undecided, 
what  is  done? 

124.  What  is  an  ohmmeter?      Sketch  the  Sage 
ohmmeter  connections. 

125.  Why  is  an  ohmmeter  called  direct  reading? 

126.  Can    a    slide    wire    bridge    be    made  direct 
reading?     If  so,  how? 

127.  Describe  the  method  of  making  a  measure- 
ment on  the  Sage  ohmmeter. 

128.  Give  the  rule  applied  to  slide  wire  bridge 
resistance  measurements. 

129.  In  how  many  ways  can  insulation  resist- 
ance be  tested?     Name  them. 

130.  What  is  the  objection  to  the  bridge,  bell 
and  magneto  methods. 

131.  For  what   duty   are   these   devices  better 
adapted? 

132.  How  great  a  voltage  should  be  used  in  in- 
sulation testing? 

133.  Is  a  thin  air-gap  perfect  insulation  against 
low  voltages? 

134.  Is  a  thin  air-gap  perfect  insulation  against 
the  higher  voltages? 

135.  Does    high    voltage    subject    insulation    to 
breaking  down  stresses? 

136.  Are  such  stresses  not  possibly  injurious  to 
the  insulation? 

137.  Where  an  actual  arc  is  started,  can  the  in- 
sulation be  carbonized? 


106  SHOP  TESTS. 

138.  What  voltage  should  railway  motor  insu- 
lation be  able  to  stand? 

139.  If  unable  to  stand  2000  volts,  what  is  liable 
to  happen? 

"  140.  Should  other  insulation  pass  a  spark  before 
the  lightning  arrester? 

141.  Can  insulation  resistance  be  measured  with 
a  voltmeter? 

142.  On  what  important  fact  is  such  a  measure- 
ment based? 

143.  State  the  general  connections  for  making 
the  test. 

144.  Give  the  rule  for  measuring  insulation  with 
a  voltmeter. 

145.  Does  failure  of  the  meter  to  deflect  indi- 
cate perfect  insulation? 

140.  Is  there  any  such  thing  as  perfect  insula- 
tion? 

147.  "What  is  meant  by  the  insulation  drop  or 
insulation  deflection? 

148.  Are  the  meter  and  insulation  drops  related 
as  are  their  resistances? 

149.  Give  the   second   and  more   common   rule 
used  in  this  measurement? 

150.  Give  the  actual  method  of  testing  armature 
insulation. 

151.  On  a  ground  return  system  what  precaution 
must  be  taken  for  ground? 

152.  Is  it  not  important  first  to  be  sure  that 
the  test  circuit  is  O.K.? 

153.  What  is  meant  by  a  megohm'? 


MISCELLANEOUS  TESTS.  107 

154.  Describe   insulation  test  on   an   armature 
installed  in  a  car. 

155.  To  what  may  low  armature  insulation  be 
due? 

156.  Is  it  important  that  car  controller  frames 
be  well  grounded? 

157.  If  not  grounded,  what  effect  is  poor  con- 
troller inside  insulation  apt  to  have? 

158.  "What  is  the  principle  of  locating  weak  in- 
sulation with  a  voltmeter? 

159.  Can  parts  individually  well  insulated  show 
low  when  connected? 

160.  How  can  commutator  insulation  from  bar 
to  shell  be  tested? 

161.  How  can  commutator  insulation  from  bar 
to  bar  be  tested? 

162.  By  what  means  can  the  bars  become  short- 
circuited  together? 

163.  Will  baking  new  or  damp  armatures  in  an 
oven  improve  the  insulation? 

164.  On  a  slotted  conduit  system  what  precau- 
tion must  be  taken  in  connecting  the  ground  test 
line? 

165.  In  shop  work  is  the  insulation  resistance 
always  actually  expressed? 

166.  Give  rule  for  finding  voltmeter  deflection 
corresponding    to    a    certain    adopted    insulation 
standard. 

167.  Can  indcandescent  lamps  be  used  to  indi- 
cate condition  of  insulation? 

168.  Give  the  connections  and  general  applica- 
tion of  the  method. 


108  SHOP  TESTS. 

169.  On  what  does  the  number  of  lamps  in  series 
depend  ? 

170.  Is  the  lamp  test  as  severe  as  would  be  di- 
rect line  voltage? 

171.  In  case  of  zero  insulation  how  do  the  lamps 
act? 

172.  Is  their  degree  of  brilliancy  any  indication 
of  state  of  insulation? 

173.  In  shop  practice  how  are  the  lamps  and 
test  lines  mounted? 

174.  When  must  a  ground  test  line  t'  be  run  as 
in  Fig.  33? 

175.  What  is  the  result  if  this  line  is  omitted? 

176.  In  using  two  test  lines  t  and  t'  what  pre- 
caution must  be  observed? 

177.  If  not  observed  are  results  apt  to  be  mis- 
leading? 

178.  What  is  the  objection  to  testing  from  bar 
to  bar  with  a  lamp  test? 

179.  Is  a  low  voltage  to  be  recommended  in  this 
connection? 

180.  Why  is  an  alternating  voltage  to  be  pre- 
ferred for  this  work? 

181.  Why  are  bell  circuits  poorly  adapted  to 
insulation  testing? 

182.  To  what  are  their  reliable  indications  really 
limited  ? 

183.  For  what  class  of  work  are  they  most  used? 

184.  How  is  a  bell  outfit  gotten  up  for  shop  use? 

185.  Give  the  method  of  applying  the  bell  test. 

186.  Is  the  voltage  sufficient  to  ring  the  bell 
through  fine  T/ire  magnets? 


MISCELLANEOUS  TESTS.  109 

187.  This  being  the   case,   is   an   indication   of 
open-circuit  always  reliable? 

188.  What  is  a  magneto  machine?    What  kind 
of  current  does  it  generate? 

189.  About  what  voltage  do  hand  turned  mag- 
netos generate? 

190.  Can  this  be  increased  by  belting  the  ma- 
chine to  shop  shafting? 

191.  Is  not  the  machine  then  very  satisfactory 
for  insulation  testing? 

192.  What  misleading  result  does  the  machine 
give  at  times? 

193.  To  what  is  this  misleading  indication  due? 

194.  On    what    electric    railway    devices    is    it 
liable  to  obtain? 

195.  What  is  meant  by  high  voltage  insulation 
tests? 

196.  Describe   a   homemade   outfit   for  making 
such  tests. 

197.  How  can   the   high   voltage   be   indicated 
without  instruments  ? 

198.  Give  the  method  of  conducting  the  test 
with  such  an  outfit. 

199.  In  what  regard  must  extreme  care  be  taken  ? 

200.  How    can    a    satisfactory    transformer    be 
homemade  ? 

201.  What  is  the  object  of  a  bar  to  bar  test  on 
an  armature? 

202.  What  is  the  common  method  of  conducting 
such  a  test? 

203.  What   does  a   variation   in   coil   resistance 
indicate  ? 


110  SHOP  TESTS. 

204.  Name  some  of  the  irregularities  that  a  coil 
may  have. 

205.  Is  it  customary  to  measure  the  absolute 
resistance  of  the  coils? 

206.  Give   a   good   device   for  introducing  test 
current  to  the  armature. 

207.  Is  it  important  to  keep  the  current  the 
same  for  two  readings? 

208.  To  what  may  an  abnormally  high  deflec- 
tion be  due? 

209.  To  what  may  an  abnormally  low  deflection 
be  due? 

210.  How  must   the   cause   of   discrepancy   be 
located? 

211.  In  case  of  an  open-circuited  coil,  what  is 
the  indication? 

212.  Can  water  resistance  be  used  to  regulate 
the  current? 

213.  How  is  the  current  affected  by  the  heating 
of  the  water? 

214.  Where  constant  voltage  is  available  is  an 
ammeter  needed  ? 

215.  If  the  test  is  prolonged  will  the  standard 
deflection  increase? 

216.  Why  will  not  this  increase  be  misleading 
to  the  tester? 

217.  Is  it  desirable  to  use  the  least  current  that 
will  give  a  satisfactory  deflection? 

218.  Must  the  rating  of  the  armature  and  in- 
struments be  considered? 

219.  Must  the  variable  resistance  be  selected  to 
suit  the  test  current? 


MISCELLANEOUS  TESTS.  Ill 

220.  Give   the   rule   for  calculating  the  proper 
regulating  resistance. 

221.  Give  the  rule  for  finding  the  current  such 
that  the  drop  on  a  coil  will  not  exceed  the  capacity 
of  the  voltmeter  used. 

222.  How  can  the  approximate  coil  resistance 
be  found  without  testing? 

223.  How  can  maximum  armature  current  be 
calculated  from  its  horse  power? 

224.  How  can  it  be  calculated  from  the  size  of 
the  wire  in  the  coil? 

225.  Must  the  method  of  connecting  the  coils 
be  considered? 

226.  Why  is  the  total  current  taken   as  twice 
that  allowed  for  the  wire? 

227.  Describe  the  lead  to  bar  test  and  state  its 
object. 

228.  What  conditions  will  cause  high  deflections? 

229.  What  appearance  may  suggest  the  neces- 
sity of  this  test? 

230.  Describe  the  telephone  method  of  testing 
an  armature. 

231.  What  source  of  current  is  generally  used? 

232.  Of  what  advantage  is  an  alternating  current 
or  a  current  interrupter? 

,  233.  What  two  influences  act  on  the  telephone 
coil? 

234.  What  is  the  indication  of  an  open  circuited 
coil? 

235.  How  is  a  short  circuited  coil  indicated? 

236.  Can  a  differential  voltmeter  be  applied  to 
this  armature  test? 


112  SHOP  TESTS. 

237.  Give  the  method  and  general  connections 
of  the  test. 

238.  Has  the  direction  of  the  voltmeter  deflec- 
tion any  significance? 

239.  What  is  the  modern  method  of  testing  for 
armature  winding  faults? 

240.  Describe  the  construction  of  the  test  shoe. 

241.  What  kind   of  e.m.f.   is   required   for  the 
transformer  test? 

242.  State  briefly  the  theory  of  the  test. 

243.  Has   each    slot    conductors    belonging     to 
both  halves  of  the  armature? 

244.  Primarily  may  the  test  shoe  coil  be  con- 
sidered as  a  choke  coil? 

245.  Does  a  short  circuit  increase  the  choke  coil 
current  ? 

246.  Can  this  increase  be  made  to  indicate  the 
existence  of  the  fault? 

247.  What  is  the  more  usual  method  of  indicating 
the  existence  of  fault? 

248.  In  how  many  slots  will  the  vibrator  show 
maximum  vibration? 

249.  What  is  the  objection  to  an  air  gap  between 
the  shoe  and  armature? 

250.  What  would  be  the  best  shoe  construction 
to  use? 

251.  Would  such  construction  be  consistent  with 
demands  for  lightness? 

252.  Describe  a  lathe  test  equivalent  to  the  a.c. 
transformer  test. 

253.  Can  the  shoe  be  wound  with  either  coarse 
or  fine  wire? 


MISCELLANEOUS  TESTS.  113 

254.  What  is  meant  by  the  spinning  test? 

255.  Name  some  of  the  conditions  revealed  by  it. 

256.  Give  a  sketch  of  the  connections  used. 

257.  What  is  the  advantage  of  making  a  field 
terminal  the  trolley  end? 

258.  Why  should  the  armature  bearing  caps  be 
screwed  down  tight? 

259.  What  is  the  indication  of  an  eccentric  com- 
mutator?    A  bent  shaft? 

260.  What  is  the  symptom  of  an  unbalanced 
core?     High  bar?     End  play? 

261.  How  should  radial  brushes  rest  on  the  com- 
mutator? 

262.  What  does  heel  or  toe  contact  indicate? 

263.  How  many  bars   should   be   included  be- 
tween inside  brush  edges? 

264.  Why  do  grounded  field  coils  sometimes  pass 
the  truck  spinning  test? 

265.  State  the  advantage  of  using  standard  con- 
nections in  the  test. 

266.  Either  of  what  two  faults  can  cause  reverse 
rotation  ? 

267.  What  polarity  have  the  single  A's  in  mod- 
ern controllers? 

268.  Knowing  this,  is  the  information  given  by 
the  spinning  test  of  any  use  in  deciding  how  to 
connect  motors  to  turn  so  as  to  move  a  car  in  a 
certain  direction? 

269.  How  are  the  directions  of  the  car  and  the 
top  of  the  armature  related? 

270.  What   should   be   the   relative   polarity  of 
motor  poles? 


114  SHOP  TESTS. 

271.  How  can  this  polarity  be  tested  with  a  nail? 

272.  What  is  the   advantage  of  removing  the 
armature  in  the  test? 

273.  What  causes  the  nail  to  take  certain  set 
positions? 

274.  Is  the  test  applicable  to  motors  of  any  num- 
ber of  poles? 

275.  Is  a  car  starting  coil  alive  during  normal 
operation? 

276.  What  happens  if  it  becomes  dead  grounded? 

277.  Why  is  it  customary  to  ground  controller 
frames  ? 

278.  What    may    happen    if    they    are    poorly 
grounded? 

279.  Can  charged  or  grounded  conductors  be  lo- 
cated with  a  voltmeter? 

280.  Give  the  rule  for  locating  a  ground  with  a 
voltmeter. 

281.  Give  the  rule  for  locating  a  charged  part 
with  a  voltmeter. 

282.  Name  some  conditions  under  which  pas- 
sengers get  shocked. 

283.  Name  a  condition  likely  to  shock  the  con- 
ductor. 

284.  In  testing  for  ground,  what  does  a  full  line 
deflection  mean? 

285.  In  testing  for  a  charged  part  what  does 
such  a  deflection  mean? 

286.  Why  are   air  compressors   and   governors 
insulated  from  ground? 

287.  Can  a  coal  stove  become  charged  as  the 


MISCELLANEOUS  TESTS.  115 

result  of  contact  with  a  live  conductor.   How  would 
such  a  charge  be  located? 

288.  Under  what  other  condition  can  the  stove 
cause  a  shock? 

289.  May  oak  be  called  a  good  insulator  against 
trolley  voltage? 

290.  How  can  the  correctness  of  instruments  be 
checked? 

291.  What  precautions  must  be  taken  in  reading 
a  deflection? 

292.  How  should  instruments  be  shielded  from 
excessive  vibrations? 

293.  Should  instruments  be  selected  to  suit  the 
work  in  hand? 

294.  What 'is  meant  by  artificial  respiration? 
296.  Describe  the  artificial  respiration  method 

of  reviving  shocked  persons.    Give  a  simple  treat- 
ment for  electric  burns. 


INDEX. 

References  are  to  paragraphs  excepting  when  otherwise 
specified. 

A. 

Aid  to  Injured Chapter  VI 

Ammeter-Measuring  Current   1 

Two  in  Parallel 2 

Shunted 3,  4,  5 

Ammeter- Resistance  Method  of  Measuring  Voltage.  .      16 

Ammeter-Shunted,  constant  for Rule  1 

Armature  Tests 50  to  72 

Bar  to  Bar  Test 50 

High  Reading 52 

Low  Reading 52 

Open  Circuit 53,  69 

Bearing 67 

Brushes 68 

Connections 71 

Differential  Voltmeter 62 

Field  Connections 72 

Ground 61,  70 

Lathe  Test 65 

Nail  Test 72 

Shaft  Test 67 

Short  Circuit 59.  60.  64   65,  69 

Spinning  Test 66 

Transformer  Test 63 

B. 

Bar-to-bar  Test  for  Armatures 50  to  69 

Bar-to-bar  Test  High  Reading .   52 

Bar-to-bar  Test  Low  Reading 52 

Bar-to-bar  Test  Open  Circuit 53-69 

Bar  to  Lead  Test 58 

117 


118  INDEX. 

Bearing 67 

Bell  Circuit  Test  for  Insulation  .      47 

Bridge,  Wheatstone       32.  33 

Brushes 68 

Burns 77 

C 

Calculation  of  Constant  for  Multiplier Rule  6 

Charged  Parts,  Test  for 73 

Connections 7] 

Constants : 

Of  Shunted  Ammeter Rule  1 

Of  Voltmeter  and  Multiplier Rule  6 

Of  Voltmeter  and  Multiplier Rule  8 

Current  Measurement Chapter  I 

Drop  Method Rule  2 

Parallel  Ammeter  Method 2 

Shunted  Ammeter  Method . .    3  4,  5,  6 

Single  Ammeter  Method ] 

Voltmeter  Method 7 

Watt-hour  Meter  Method 9 

Wattmeter  Method 8 

Current-Resistance  Method  of  Measuring  Resistance. . .     29 

D. 

Differential  Voltmeter  Test  for  Armatures 62 

Differential    Voltmeter    Method    of    Measuring    Re- 
sistance  30.  31 

Drop- Method  Measuring  Current Rule  2 

Drop-Method  Measuring  Voltage Rule  9 

E. 

Experimental  Determination  of  Multiplier  for  Volt- 
meter   Rule  8 

F. 

Fall  of  Potential  Method  of  Measuring  Resistance    .  .     28 
Field  Connection  Test . .  72 


INDEX.  119 

G. 

Grounded  Parts,  Test  for 74 

Ground  Test  for  Armatures 61-70 

H. 
High  Voltage  Test  for  Insulation 49 

I. 

Insulation Chapter  IV 

Bell  Circuit  Test  for 47 

High  Voltage  Test  for 49 

Lamp  Circuit  Test  for 44,  45,  46 

Magneto  Test  for 48 

Voltmeter  Method  of  Measuring 38  to  43 

L. 

Lamp  Circuit  Test  for  Insulation 44  to  46 

Lamp  Method  of  Measuring  Voltage 18 

Lathe  Test  for  Armatures 65 

Lead  to  Bar  Test  for    Armatures 58 

M. 

Magneto  Test  for  Insulation 48 

Miscellaneous  Tests Chapter  V 

Multiple  Resistance  (see  Parallel  Resistance). 

Multiplier  for  Voltmeter 13 

Determination  by  Calculation 14 

Determination  by  Experiment 15 

Multiplier  for  Voltmeter,  Calculation  of Rule  6 

Experimental  Determination  of Rule  8 

N. 
Nail  Test 72 

O. 

Ohmmeter ...  ,34 


120  INDEX. 

P. 

Parallel  Ammeters 2 

Parallel  Resistance 21  to  26 

Parallel  Resistance Rules  7,  10 

Of  two  Equal  Resistances Rule  1 1 

Of  any  Two  Resistances Rule  12 

Of  any  Unequal  Resistances Rule  13 

To  be  added  to  produce  given  result : 

Two  Resistances Rule  14 

Any  Number  Resistances Rule  15 

Precaution  for  Testing 75 

Proportion  Lines  (Voltage  Measurement) 17 

R. 

Resistance- Ammeter  Method  of  Measuring  Voltage. .     16 
Resistance-Current  Method  of  Measuring  Resistance  .     29 

Resistance  Measurement Chapter  III 

Bridge  Method 32 

Current-Resistance  Method 29 

Differential  Voltmeter  Method 30,  31 

Fall  of  Potential  Method 28 

Ohmmeter 34 

Parallel  Resistance 21  to  26 

Resistance,  Definition 19 

Series  Resistance 20 

Voltmeter  Method 27 

Wheatstone  Bridge 32,  33 

Reviving  Shocked  Persons 76 

S. 

Series  Resistance 20 

Shaft 67 

Shocks  and  Burns 76,  77 

Short -Circuit  Test  for  Armatures 59-60-64-65-69 

Shunted  Ammeter 3,  4,  5 

Shunted  Ammeter,  Constant  of Rule  1 

Spinning  Test  for  Armatures 66 


INDEX.  121 

T. 

Testing — General  Precautions 75 

Transformer  Test  for  Armatures 63 

V. 

Voltage-Maximum  for  Voltmeters  in  Series Rule  5 

Voltage-Drop  Method  of  Measurement Rule  9 

Voltage,  Measured  with  Watt-hour  Meter Rule  4 

Voltage,  Measured  with  Wattmeter Rule  3 

Voltage  Measurement Chapter  II 

Ammeter- Resistance  Method 16 

Determination  of  Multipliers: 

(a)  By  Calculation 14 

(b)  By  Experiment 15 

Lamp  Method 18 

Proportion  Lines 17 

Series  Voltmeter  Method 12 

Single  Voltmeter  Method 10,  11 

Voltmeter  and  Multiplier 13 

Voltmeter-Calculation  of  Multiplier Rule  6 

Experimental  Determination Rule  8 

Voltmeter,  Differential  Test  for  Armatures 62 

Voltmeter,  Differential,  Use  in  Measuring  Resistance  30,  31 

Voltmeter  Method  Measuring  Current 7 

Voltmeter  Method  of  Measuring  Insulation 38  to  43 

Voltmeter  Method  of  Measuring  Resistance 27 

Voltmeter  and  Multiplier 13 

Voltmeter,  Single 10,  1 1 

Two  in  Series 12 

W. 

Watt-hour  Meter  Method  of  Measuring  Current 9 

Wattmeter  Method  of  Measuring  Voltage Rule  3 

Watt-hour  Meter  Method  of  Measuring  Voltage . . .  Rule  4 

Wattmeter  Method  of  Measuring  Current 8 

Wheatstone  Bridge 32,  33 


rorm  L-9-15m-7,'32 


UNIVERSITY  of  CAL1FORNU 
L.US  ANGBLJBJ3 


